diff --git a/.changeset/26690-translation-books.md b/.changeset/26690-translation-books.md
new file mode 100644
index 00000000000..dc63c26821d
--- /dev/null
+++ b/.changeset/26690-translation-books.md
@@ -0,0 +1,34 @@
+---
+'@objectstack/spec': minor
+'@objectstack/rest': minor
+'@objectstack/runtime': minor
+'@objectstack/cli': minor
+'@objectstack/lint': minor
+'@object-ui/app-shell': minor
+'@object-ui/i18n': minor
+---
+
+feat(spec,rest,cli): a doc book's title and section titles are translatable — `books.<name>` in the translation bundle
+
+Clause-②: yes (widening)
+Metadata-protocol: added
+
+A package that ships its docs in two languages showed a half-translated manual: each doc's body followed the reader's locale (`doc.translations`, `<name>.<locale>.md`), but the book title and every sidebar section title stayed in the authoring language. The book's own inline `translations` map was retired in 17.0.0 because nothing read it, and the translation bundle had no key for a book.
+
+- **`@objectstack/spec`:** `TranslationData` (and the registered `translation` item and the platform bundle, which share its groups) gains `books`, keyed by the book's `name`: `label`, `description`, and `groups.<key>.label` for each group by its stable `key` (strict: any other key is refused; `name` / `title` point at `label`, and `sections` / `chapters` / `toc` point at `groups`; the singular group spelling `book` points at `books`). A new translator, `translateBook`, overlays them and is registered in the metadata-document dispatch, so `GET /meta/book` (list and item) is served in the request's locale. A key the bundle does not carry keeps the authored string. The rejection for the retired inline `translations` map on a book now names the bundle keys to move the strings to.
+- **`@objectstack/rest`:** `GET /meta/book/:name/tree` translates the book it resolves through the same dispatch before rendering the spine, so the tree, the book list and the console's docs sidebar agree. `MetaBookTreeSources` gains `resolveI18nService()`, which both transports supply.
+- **`@objectstack/runtime`:** the dispatcher's book-tree route supplies the same i18n service, so it answers the localized tree `RestServer` answers.
+- **`@objectstack/cli`:** `os i18n extract` scaffolds `books.<name>.label` / `.description` and `books.<name>.groups.<key>.label` from the stack's `books[]`, and `os i18n check` / `os lint` coverage report an untranslated one as `i18n/missing-book`.
+- **`@objectstack/lint`:** `translation-target-unknown` resolves `books.<name>` against the declared books (a sibling package's included, and a package id for its implicit book) and `books.<name>.groups.<key>` against that book's group `key`s, so a mistyped book name or group key is an error instead of a translation nothing reads.
+- **`@object-ui/i18n`:** the console's language loader recognises `books` as a translation group, so a served document that carries only book translations is namespaced like any other.
+
+Example (`zh-CN` bundle of an English-authored package):
+
+```ts
+books: {
+  crm_manual: {
+    label: '使用手册',
+    groups: { start: { label: '快速上手' }, pipeline: { label: '销售管道' } },
+  },
+},
+```
diff --git a/content/docs/protocol/kernel/i18n-standard.mdx b/content/docs/protocol/kernel/i18n-standard.mdx
index 7979b59f9d2..56672622c34 100644
--- a/content/docs/protocol/kernel/i18n-standard.mdx
+++ b/content/docs/protocol/kernel/i18n-standard.mdx
@@ -406,8 +406,8 @@ Three rules the shape enforces, all of them closed since #4001:
 
 - **The top level is the declared group set and nothing else** — `objects`,
   `picklists`, `apps`, `messages`, `globalActions`, `dashboards`, `datasets`,
-  `reports`, `pages`, `flows`, `positions`, `permissions`, `metadataForms`,
-  `settingsCommon`. A group invented beside them
+  `reports`, `pages`, `flows`, `positions`, `permissions`, `books`,
+  `metadataForms`, `settingsCommon`. A group invented beside them
   (`"account"`, `"fields"`, `"list"`, `"industries"`) is rejected **by name**
   at both authoring doors, with the group to use instead named in the
   rejection.
@@ -496,7 +496,7 @@ what the overlay writes back as the node's `title`.
 The contract is `t(key, locale, params?)` — the target locale is always passed
 explicitly, and the key itself is returned when no translation is found (there
 is no `defaultValue` option). The key is a **dot path into `TranslationData`**,
-so it starts at one of the fourteen declared groups.
+so it starts at one of the fifteen declared groups.
 
 ```typescript
 const i18n = kernel.getService('i18n');
diff --git a/content/docs/references/api/protocol.mdx b/content/docs/references/api/protocol.mdx
index fd5d355ab29..ec6bd77e93b 100644
--- a/content/docs/references/api/protocol.mdx
+++ b/content/docs/references/api/protocol.mdx
@@ -1783,6 +1783,7 @@ The published metadata item body, opaque by ruling (1C). Shape is the item's own
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
 
diff --git a/content/docs/references/system/book.mdx b/content/docs/references/system/book.mdx
index 48e785108dc..0cab1ce8a51 100644
--- a/content/docs/references/system/book.mdx
+++ b/content/docs/references/system/book.mdx
@@ -47,7 +47,7 @@ const result = BookSchema.parse(data);
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **name** | `string` | ✅ | Book name (namespace prefix recommended, like every metadata name) |
-| **label** | `string` | optional | Display title |
+| **label** | `string` | optional | Display title — translated by `books.<book>.label` in the translation bundle |
 | **description** | `string` | optional |  |
 | **slug** | `string` | optional | Portal URL segment; defaults to name sans prefix |
 | **icon** | `string` | optional |  |
@@ -67,8 +67,8 @@ const result = BookSchema.parse(data);
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **key** | `string` | ✅ | Stable group key (used by overrides, deep links, explicit `doc.group`) |
-| **label** | `string` | ✅ | Section title — first-class, i18n-homed |
-| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it. The book tree endpoint and the docs portal render `label` / `description` verbatim in every locale, so a localized book shipped its authoring-locale strings to every reader. Delete the key. NOTE the near neighbour that DOES work: `doc.translations` is live and read on every doc render path — localize the docs themselves, and the portal picks the reader's locale up from there. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
+| **label** | `string` | ✅ | Section title — first-class, i18n-homed: translated by `books.<book>.groups.<key>.label` in the translation bundle |
+| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it, so a localized book shipped its authoring-locale strings to every reader. Delete the key and move each locale's strings into the translation bundle: `books.<book>.label` / `.description` and `books.<book>.groups.<group_key>.label` (`os i18n extract` seeds them). The book list, the book tree and the docs sidebar resolve those per reader locale. A doc's own title and body still localize through `doc.translations`. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
 | **order** | `number` | optional | Order of THIS group within the book |
 | **include** | `string \| { tag: string }` | optional | Rule that derives membership (glob or tag) |
 | **package** | `string` | optional | Scope the rule to a package id (default: the book package; cross-package via ADR-0048) |
@@ -115,8 +115,8 @@ Type: `'public'`
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
 | **key** | `string` | ✅ | Stable group key (used by overrides, deep links, explicit `doc.group`) |
-| **label** | `string` | ✅ | Section title — first-class, i18n-homed |
-| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it. The book tree endpoint and the docs portal render `label` / `description` verbatim in every locale, so a localized book shipped its authoring-locale strings to every reader. Delete the key. NOTE the near neighbour that DOES work: `doc.translations` is live and read on every doc render path — localize the docs themselves, and the portal picks the reader's locale up from there. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
+| **label** | `string` | ✅ | Section title — first-class, i18n-homed: translated by `books.<book>.groups.<key>.label` in the translation bundle |
+| **translations** | `never` | optional | [REMOVED] Inline `translations` on a book (and on a book group) was removed in @objectstack/spec 17.0.0 (ADR-0049) — no resolver ever read it, so a localized book shipped its authoring-locale strings to every reader. Delete the key and move each locale's strings into the translation bundle: `books.<book>.label` / `.description` and `books.<book>.groups.<group_key>.label` (`os i18n extract` seeds them). The book list, the book tree and the docs sidebar resolve those per reader locale. A doc's own title and body still localize through `doc.translations`. This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade. |
 | **order** | `number` | optional | Order of THIS group within the book |
 | **include** | `string \| { tag: string }` | optional | Rule that derives membership (glob or tag) |
 | **package** | `string` | optional | Scope the rule to a package id (default: the book package; cross-package via ADR-0048) |
diff --git a/content/docs/references/system/translation.mdx b/content/docs/references/system/translation.mdx
index beafb946cec..4224f0eaa11 100644
--- a/content/docs/references/system/translation.mdx
+++ b/content/docs/references/system/translation.mdx
@@ -185,6 +185,7 @@ Platform translation data — the per-app groups plus the platform-only `setting
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
@@ -295,6 +296,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `PlatformTranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `PlatformTranslationData.metadataForms[string]`
 
 | Property | Type | Required | Description |
@@ -344,6 +353,7 @@ Served translation data — the platform groups the browser reads, without the s
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **settings** | `Record<string, { title?: string; description?: string; groups?: Record<string, object>; keys?: Record<string, object>; … }>` | optional | Settings manifest translations keyed by namespace |
 
@@ -453,6 +463,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `ServedTranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `ServedTranslationData.settingsCommon`
 
 | Property | Type | Required | Description |
@@ -564,6 +582,7 @@ Per-app translation data for objects, apps, and UI messages
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 
@@ -673,6 +692,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `TranslationData.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `TranslationData.metadataForms[string]`
 
 | Property | Type | Required | Description |
@@ -743,6 +770,7 @@ One locale of translations — the `translation` metadata type
 | **flows** | `Record<string, { label?: string; successMessage?: string; errorMessage?: string; screens?: Record<string, object>; … }>` | optional | Screen-flow translations keyed by flow name |
 | **positions** | `Record<string, { label?: string; description?: string }>` | optional | Position translations keyed by position name |
 | **permissions** | `Record<string, { label?: string; description?: string }>` | optional | Permission-set translations keyed by permission-set name |
+| **books** | `Record<string, { label?: string; description?: string; groups?: Record<string, object> }>` | optional | Doc-book translations keyed by book name |
 | **metadataForms** | `Record<string, { label?: string; description?: string; sections?: Record<string, object>; fields?: Record<string, object> }>` | optional | Translations for metadata-type configuration forms keyed by metadata type |
 | **settingsCommon** | `{ sourceLabels?: object }` | optional | Cross-namespace Settings UI strings |
 | **locale** | `string` | ✅ | BCP-47 locale this item translates (e.g. "zh-CN") |
@@ -862,6 +890,14 @@ Translation data for a single object
 | **label** | `string` | optional | Translated permission-set label — overlays the set's own `label` |
 | **description** | `string` | optional | Translated permission-set description — overlays the set's own `description` |
 
+### Nested Shape: `TranslationItem.books[string]`
+
+| Property | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| **label** | `string` | optional | Translated book title — overlays the book's own `label` |
+| **description** | `string` | optional | Translated book description — overlays the book's own `description` |
+| **groups** | `Record<string, { label: string }>` | optional | Group (section) translations keyed by the group `key` |
+
 ### Nested Shape: `TranslationItem.metadataForms[string]`
 
 | Property | Type | Required | Description |
diff --git a/content/docs/ui/doc-pages.mdx b/content/docs/ui/doc-pages.mdx
index 601777e831a..fa8e86f55e1 100644
--- a/content/docs/ui/doc-pages.mdx
+++ b/content/docs/ui/doc-pages.mdx
@@ -302,11 +302,34 @@ expands to *the rest* — every doc the group's rule would have claimed but that
 names, in `order`-then-label sequence. An entry can also be an object to attach a `label`
 override, a `badge` or an `icon`, or to point at an external `href` instead of a doc.
 
+#### Translating the spine
+
+A book's title, its description and each group's section title translate through the
+translation bundle, like an app's label and its navigation labels — keyed by the book's
+`name` and each group's `key`:
+
+```typescript
+translations: [{
+  'zh-CN': {
+    books: {
+      crm_docs: {
+        label: 'CRM 文档',
+        groups: { overview: { label: '概览' }, guides: { label: '使用指南' }, admin: { label: '管理' } },
+      },
+    },
+  },
+}],
+```
+
+`GET /meta/book`, the book tree and the docs sidebar serve the reader's locale; a key the
+bundle does not carry keeps the authored string. `os i18n extract` seeds the keys, and
+`os i18n check` reports a missing one as `i18n/missing-book`. The docs themselves localize
+through `doc.translations` and `<name>.<locale>.md` variants.
+
 <Callout type="warn">
 Inline `translations` on a **book** or a **book group** is rejected: no resolver ever read
-it, so a localized spine shipped its authoring-locale strings to every reader. The near
-neighbour that *does* work is `doc.translations`, read on every doc render path — localize
-the docs themselves.
+it, so a localized spine shipped its authoring-locale strings to every reader. Move those
+strings into the bundle's `books` group above.
 </Callout>
 
 ### How `doc` and `book` compose
diff --git a/content/docs/ui/translations.mdx b/content/docs/ui/translations.mdx
index 56b18df8b84..6a572ff3959 100644
--- a/content/docs/ui/translations.mdx
+++ b/content/docs/ui/translations.mdx
@@ -89,12 +89,13 @@ export default defineStack({
 | Screen-flow wizards (flow label, screen headings and body text, screen field copy) | `flows.<flow>.label` / `flows.<flow>.screens.<node_id>.title` / `.description` / `.fields.<field>.label` / `.placeholder` / `.inlineHelpText` — a screen field's help text is `inlineHelpText`, the key the field itself declares, not `help`; see the boundary note below |
 | Position label / description | `positions.<name>.label` / `description` — keyed by the position's `name`; overlaid on every served position, which is the list the console's position picker draws |
 | Permission-set label / description | `permissions.<name>.label` / `description` — the group is spelled as the stack spells the collection (`permissions[]`), keyed by the set's `name` |
+| Doc book title, description and section titles | `books.<name>.label` / `description` / `groups.<key>.label` — keyed by the book's `name` and each group's `key`; overlaid on every served book and on the book tree, which is what the docs sidebar draws. A doc's own title and body localize through `doc.translations` instead |
 | Global actions, messages | `globalActions`, `messages` |
 | Settings UI shell copy (the source badge on a settings row) | `settingsCommon.sourceLabels.<layer>` — the per-namespace settings copy under `settings` is **platform-only**: an app bundle carrying it is refused by name, and the platform's own strings are translated in `@objectstack/service-settings`'s bundle |
 | A label written as an inline locale map (`label: { en: 'Members', 'zh-CN': '成员' }`) | Nowhere — it is written on the metadata and resolved at render time; see **Current boundaries** below |
 
 The metadata types resolved per request are **object, view, action, app,
-dashboard, dataset, report, page, picklist, position and permission** — a field's labels are translated as part of
+dashboard, dataset, report, page, picklist, position, permission and book** — a field's labels are translated as part of
 its object document, and so are the labels of any actions the object declares
 inline
 (#3370). Before that, an object document went out with its authored action
diff --git a/docs/audits/2026-07-unknown-key-strictness-ledger.counts/system.md b/docs/audits/2026-07-unknown-key-strictness-ledger.counts/system.md
index eb3d3289225..c9ba2459292 100644
--- a/docs/audits/2026-07-unknown-key-strictness-ledger.counts/system.md
+++ b/docs/audits/2026-07-unknown-key-strictness-ledger.counts/system.md
@@ -19,4 +19,4 @@ hand-patch a number here** — fix the code or the verdict and regenerate.
 
 | Dir | Sites |
 |---|---|
-| `system/` | 384 |
+| `system/` | 386 |
diff --git a/examples/app-showcase/src/system/translations/index.ts b/examples/app-showcase/src/system/translations/index.ts
index b00559fb8f3..f890fc39408 100644
--- a/examples/app-showcase/src/system/translations/index.ts
+++ b/examples/app-showcase/src/system/translations/index.ts
@@ -1365,5 +1365,19 @@ export const ShowcaseTranslationBundle = {
       },
       showcase_hours_by_status_chart: { label: '按状态统计工时（图表）', description: '以柱状图展示按状态统计的预计工时。' },
     },
+    // The docs book: its title and the docs sidebar's section titles, keyed by
+    // the book's `name` and each group's `key`. The docs themselves localize
+    // through `doc.translations` and `<name>.zh-CN.md` variants.
+    books: {
+      showcase_manual: {
+        label: '示例手册',
+        description: '这个综合示例工作区里的一切。',
+        groups: {
+          start: { label: '入门' },
+          tour: { label: '导览' },
+          guides: { label: '指南' },
+        },
+      },
+    },
   },
 };
diff --git a/packages/cli/src/utils/i18n-coverage.ts b/packages/cli/src/utils/i18n-coverage.ts
index 5afbd13d342..d6f085d71bb 100644
--- a/packages/cli/src/utils/i18n-coverage.ts
+++ b/packages/cli/src/utils/i18n-coverage.ts
@@ -73,6 +73,7 @@ export interface CoverageIssue {
     | 'flow'
     | 'position'
     | 'permission'
+    | 'book'
     | 'metadataForm';
   /** Human-readable explanation. */
   message: string;
@@ -385,6 +386,10 @@ const COVERAGE_SOURCE: Record<ExpectedEntry['source'], CoverageIssue['source']>
   // `i18n/missing-position` / `i18n/missing-permission`.
   position: 'position',
   permission: 'permission',
+  // Doc-book spine copy (`books.<b>.label` / `.description`,
+  // `books.<b>.groups.<key>.label`) — the title and section titles of a docs
+  // sidebar, reported as `i18n/missing-book` (#26690).
+  book: 'book',
   metadataType: 'metadataForm',
   metadataFormSection: 'metadataForm',
   metadataFormField: 'metadataForm',
@@ -409,6 +414,7 @@ const SOURCE_NOUN: Record<CoverageIssue['source'], string> = {
   flow: 'Flow',
   position: 'Position',
   permission: 'Permission set',
+  book: 'Book',
   metadataForm: 'Metadata form',
 };
 
@@ -454,6 +460,7 @@ const SOURCE_SURFACE: Record<CoverageIssue['source'], string> = {
   flow: 'flow screens, approval steps, toasts and refusals',
   position: 'positions',
   permission: 'permission sets',
+  book: 'doc books',
   metadataForm: 'metadata forms',
 };
 
diff --git a/packages/cli/src/utils/i18n-extract.ts b/packages/cli/src/utils/i18n-extract.ts
index 887c3182bf2..323e1975f90 100644
--- a/packages/cli/src/utils/i18n-extract.ts
+++ b/packages/cli/src/utils/i18n-extract.ts
@@ -126,6 +126,8 @@
  *       (see `authorWarnedTranslationGroups`)
  *   positions.<position>.label / .description
  *   permissions.<permission_set>.label / .description
+ *   books.<book>.label / .description
+ *   books.<book>.groups.<group_key>.label   (a doc book's section title, #26690)
  *   metadataForms.<type>.label / .description
  *   metadataForms.<type>.sections.<section>.label / .description
  *   metadataForms.<type>.fields.<dotPath>.label / .helpText / .placeholder
@@ -232,6 +234,7 @@ export interface ExpectedEntry {
     | 'flow'
     | 'position'
     | 'permission'
+    | 'book'
     | 'metadataType'
     | 'metadataFormSection'
     | 'metadataFormField';
@@ -1638,6 +1641,9 @@ export function collectExpectedEntries(
   // ── Security catalog (`positions.<name>.…`, `permissions.<name>.…`) ─
   walkSecurityCatalog(config, out);
 
+  // ── Doc books (`books.<name>.…`) ──────────────────────────────────
+  walkBooks(config, out);
+
   // ── Pages + their `page:header` copy ──────────────────────────────
   const pages: any[] = Array.isArray(config?.pages) ? config.pages : [];
   for (const page of pages) {
@@ -1953,6 +1959,34 @@ function walkSecurityCatalog(config: any, out: ExpectedEntry[]): void {
   }
 }
 
+// ─── Doc books (`books.<name>.…`) ──────────────────────────────────────
+
+/**
+ * [#26690] Emit a doc book's spine copy: `books.<book>.label` /
+ * `.description` and `books.<book>.groups.<key>.label`, the keys
+ * `translateBook` reads. A group is addressed by its stable `key`, never its
+ * position. The book's own label and description are optional on the schema,
+ * so they are recorded with {@link pushOptional}; a group's label is required,
+ * so it is the author's literal ({@link pushEntry}).
+ */
+function walkBooks(config: any, out: ExpectedEntry[]): void {
+  const books: any[] = Array.isArray(config?.books) ? config.books : [];
+  for (const book of books) {
+    if (!book || typeof book !== 'object') continue;
+    const name = book.name;
+    if (typeof name !== 'string' || name.length === 0) continue;
+    pushOptional(out, ['books', name, 'label'], book.label, 'book');
+    pushOptional(out, ['books', name, 'description'], book.description, 'book');
+    const groups: any[] = Array.isArray(book.groups) ? book.groups : [];
+    for (const group of groups) {
+      if (!group || typeof group !== 'object') continue;
+      const key = group.key;
+      if (typeof key !== 'string' || key.length === 0) continue;
+      pushEntry(out, ['books', name, 'groups', key, 'label'], group.label, 'book');
+    }
+  }
+}
+
 // ─── Screen flows (`flows.<flow>.screens.<node_id>.…`) ─────────────────
 
 /**
diff --git a/packages/cli/test/i18n-book-walk.test.ts b/packages/cli/test/i18n-book-walk.test.ts
new file mode 100644
index 00000000000..d352c245df4
--- /dev/null
+++ b/packages/cli/test/i18n-book-walk.test.ts
@@ -0,0 +1,88 @@
+// Copyright (c) 2026 ObjectStack contributors. Apache-2.0 license.
+//
+// `books.<name>.…` — the extractor walks a doc book's translation group.
+//
+// A book's title and section titles frame every page of a docs sidebar, and
+// `translateBook` overlays them from the bundle. Without this walk
+// `os i18n extract` would scaffold no key for them and `os i18n check` would
+// measure a debt of zero while the sidebar shipped in the authoring language.
+
+import { describe, it, expect } from 'vitest';
+import { collectExpectedEntries, extractTranslations } from '../src/utils/i18n-extract.js';
+import { computeI18nCoverage } from '../src/utils/i18n-coverage.js';
+import { TranslationDataSchema } from '@objectstack/spec/system';
+
+const config = () => ({
+  books: [
+    {
+      name: 'crm_manual',
+      label: 'User Manual',
+      description: 'Everything a sales rep needs',
+      groups: [
+        { key: 'start', label: 'Getting started', include: 'crm_start_*' },
+        { key: 'pipeline', label: 'Pipeline', include: 'crm_pipe_*' },
+      ],
+    },
+    // No label, no description: nothing an author wrote, so nothing is demanded.
+    { name: 'crm_admin_manual', groups: [{ key: 'setup', label: 'Setup' }] },
+  ],
+});
+
+const keys = () =>
+  collectExpectedEntries(config())
+    .filter((e) => e.path[0] === 'books')
+    .map((e) => e.path.join('.'))
+    .sort();
+
+describe('i18n extract — the `books` group', () => {
+  it('walks each book\'s label, description and every group label by its key', () => {
+    expect(keys()).toEqual([
+      'books.crm_admin_manual.description',
+      'books.crm_admin_manual.groups.setup.label',
+      'books.crm_admin_manual.label',
+      'books.crm_manual.description',
+      'books.crm_manual.groups.pipeline.label',
+      'books.crm_manual.groups.start.label',
+      'books.crm_manual.label',
+    ]);
+  });
+
+  it('seeds authored text and records an unwritten string without demanding it', () => {
+    const byKey = new Map(collectExpectedEntries(config()).map((e) => [e.path.join('.'), e]));
+    expect(byKey.get('books.crm_manual.label'))
+      .toMatchObject({ sourceValue: 'User Manual', inline: 'User Manual', source: 'book' });
+    expect(byKey.get('books.crm_manual.groups.start.label'))
+      .toMatchObject({ sourceValue: 'Getting started', inline: 'Getting started', source: 'book' });
+    const unwritten = byKey.get('books.crm_admin_manual.label');
+    expect(unwritten?.sourceValue).toBeUndefined();
+    expect(unwritten?.inline).toBeUndefined();
+  });
+
+  it('`TranslationDataSchema` accepts a bundle written at the extracted paths', () => {
+    const out = extractTranslations(config(), { locales: ['zh-CN'] });
+    const zh = out.bundles['zh-CN'] as any;
+    expect(Object.keys(zh?.books ?? {}).sort()).toEqual(['crm_admin_manual', 'crm_manual']);
+    expect(Object.keys(zh?.books?.crm_manual?.groups ?? {}).sort()).toEqual(['pipeline', 'start']);
+    expect(TranslationDataSchema.safeParse({ books: zh.books }).success).toBe(true);
+  });
+
+  it('reports an untranslated label under its own bucket', () => {
+    const report = computeI18nCoverage({
+      ...config(),
+      i18n: { supportedLocales: ['en', 'zh-CN'] },
+      translations: [
+        { 'zh-CN': { books: { crm_manual: { label: '使用手册', groups: { start: { label: '快速上手' } } } } } },
+      ],
+    });
+    const zhIssues = report.issues.filter((i) => i.locale === 'zh-CN');
+    const missing = zhIssues.map((i) => i.key);
+    expect(missing).toContain('books.crm_manual.groups.pipeline.label');
+    expect(missing).toContain('books.crm_manual.description');
+    expect(missing).toContain('books.crm_admin_manual.groups.setup.label');
+    expect(missing).not.toContain('books.crm_manual.label');
+    expect(missing).not.toContain('books.crm_manual.groups.start.label');
+    // Nothing authored ⇒ nothing to translate.
+    expect(missing).not.toContain('books.crm_admin_manual.label');
+    expect(zhIssues.find((i) => i.key === 'books.crm_manual.groups.pipeline.label')?.source).toBe('book');
+  });
+});
diff --git a/packages/lint/src/rule-explanations.ts b/packages/lint/src/rule-explanations.ts
index c6e1e73b6a9..619a8bbdcfc 100644
--- a/packages/lint/src/rule-explanations.ts
+++ b/packages/lint/src/rule-explanations.ts
@@ -3541,7 +3541,9 @@ const TRANSLATION_TARGET_UNKNOWN_EXPLANATION: RuleExplanation = {
       'stack declares. `positions.NAME` a position\'s `name` that a package of this stack ' +
       'declares, or an audience anchor (`everyone`, `guest`); `permissions.NAME` a permission ' +
       'set\'s `name` that a package of this stack declares. `reports.NAME` a report\'s `name` that ' +
-      'a package of this stack declares.',
+      'a package of this stack declares. `books.BOOK` a doc book\'s `name` that a package of this ' +
+      'stack declares, or a package id (its implicit book), and `.groups.KEY` a group `key` that ' +
+      'book declares, not its label.',
     // The terminal toast keys are held to `FLOW_TERMINAL_MESSAGE_KEYS` by the rule's test.
     '`flows.FLOW` a flow\'s machine `name`; `.successMessage` / `.errorMessage` a toast the flow ' +
       'itself authors (a bundle cannot add one); `.screens.NODE` the `id` of a `type: \'screen\'` ' +
diff --git a/packages/lint/src/validate-translation-references.test.ts b/packages/lint/src/validate-translation-references.test.ts
index 837615236d7..59b1ce6e9b1 100644
--- a/packages/lint/src/validate-translation-references.test.ts
+++ b/packages/lint/src/validate-translation-references.test.ts
@@ -4,7 +4,7 @@ import { describe, it, expect } from 'vitest';
 // The contract the object branch's coverage is pinned against (#13835) — see
 // the coverage describe block at the foot of this file. `@objectstack/spec` is
 // already a runtime dependency of this package, so the pin adds no edge.
-import { FLOW_TERMINAL_MESSAGE_KEYS, ObjectTranslationDataSchema, TranslationDataSchema } from '@objectstack/spec/system';
+import { FLOW_TERMINAL_MESSAGE_KEYS, ObjectTranslationDataSchema, TranslationDataSchema, deriveImplicitPackageBook } from '@objectstack/spec/system';
 // The schema the #18441 fold is SIZED against — see the surface pin in that
 // block. Same package, already a runtime dependency, so no new edge either.
 import { ObjectExtensionSchema } from '@objectstack/spec/data';
@@ -4513,3 +4513,63 @@ describe('validateTranslationReferences — reports', () => {
     expect(findings).toEqual([]);
   });
 });
+
+describe('validateTranslationReferences — doc books (#26690)', () => {
+  const stack = (data: Record<string, unknown>, extra: Record<string, unknown> = {}) => ({
+    books: [{
+      name: 'crm_manual',
+      label: 'User Manual',
+      groups: [
+        { key: 'start', label: 'Getting started', include: 'crm_start_*' },
+        { key: 'pipeline', label: 'Pipeline', include: 'crm_pipe_*' },
+      ],
+    }],
+    ...extra,
+    translations: [{ en: data }],
+  });
+
+  it('a declared book and its declared group keys are clean', () => {
+    expect(validateTranslationReferences(stack({
+      books: { crm_manual: { label: '使用手册', description: '说明', groups: { start: { label: '快速上手' }, pipeline: { label: '销售管道' } } } },
+    }))).toEqual([]);
+  });
+
+  it('a book name no package declares is translation-target-unknown, at error', () => {
+    const findings = validateTranslationReferences(stack({ books: { crm_manaul: { label: 'x' } } }));
+    expect(findings.map((f) => [f.rule, f.severity, f.path, f.where])).toEqual([
+      [TRANSLATION_TARGET_UNKNOWN, 'error', 'translations[0].en.books.crm_manaul', 'locale "en" · book "crm_manaul"'],
+    ]);
+    expect(findings[0].message).toContain('Did you mean "crm_manual"?');
+  });
+
+  it('a group key the book does not declare is translation-target-unknown, even when it is a group LABEL', () => {
+    const findings = validateTranslationReferences(stack({
+      books: { crm_manual: { groups: { start: { label: 'ok' }, strat: { label: 'x' }, Pipeline: { label: 'y' } } } },
+    }));
+    expect(findings.map((f) => [f.rule, f.severity, f.path])).toEqual([
+      [TRANSLATION_TARGET_UNKNOWN, 'error', 'translations[0].en.books.crm_manual.groups.strat'],
+      [TRANSLATION_TARGET_UNKNOWN, 'error', 'translations[0].en.books.crm_manual.groups.Pipeline'],
+    ]);
+    expect(findings[0].message).toContain('Did you mean "start"?');
+  });
+
+  it('resolves a book a sibling package of the artifact declares, with its group keys', () => {
+    expect(validateTranslationReferences(stack(
+      { books: { ops_guide: { label: '运维指南', groups: { deploy: { label: '部署' } } } } },
+      { packages: [{ manifest: { id: 'com.example.ops', books: [{ name: 'ops_guide', groups: [{ key: 'deploy', label: 'Deploy', include: '*' }] }] } }] },
+    ))).toEqual([]);
+  });
+
+  it("resolves the implicit per-package book: the package id, and its one group's key", () => {
+    const implicitKey = deriveImplicitPackageBook('com.example.crm').groups[0].key;
+    expect(validateTranslationReferences(stack(
+      { books: { 'com.example.crm': { label: '帮助', groups: { [implicitKey]: { label: '全部文档' } } } } },
+      { manifest: { id: 'com.example.crm' } },
+    ))).toEqual([]);
+    // … and a group key the implicit book does not carry is still an orphan.
+    expect(validateTranslationReferences(stack(
+      { books: { 'com.example.crm': { groups: { start: { label: 'x' } } } } },
+      { manifest: { id: 'com.example.crm' } },
+    )).map((f) => f.path)).toEqual(['translations[0].en.books.com.example.crm.groups.start']);
+  });
+});
diff --git a/packages/lint/src/validate-translation-references.ts b/packages/lint/src/validate-translation-references.ts
index a3540c8fe1d..16b04eea9a6 100644
--- a/packages/lint/src/validate-translation-references.ts
+++ b/packages/lint/src/validate-translation-references.ts
@@ -162,6 +162,16 @@
  * this stack declares is an orphan, `translation-target-unknown` (`error`).
  * Leaf keys below the name are the schema's (`label` / `description`, strict).
  *
+ * ── Doc books ────────────────────────────────────────────────────────────
+ *
+ * `books.<name>` resolves against `Book.name`, the name `translateBook` looks
+ * a served book up by, and `books.<name>.groups.<key>` against that book's
+ * group `key`s (#26690). A book no package declares, or a group key the book
+ * does not declare, is an orphan, `translation-target-unknown` (`error`). The
+ * implicit per-package book (`deriveImplicitPackageBook`: named by the package
+ * id, one group keyed `all`) is what the tree serves where a package authors
+ * no book of that name, so its keys resolve too.
+ *
  * ── Cross-package objects ────────────────────────────────────────────────
  *
  * A stack legitimately translates objects it does not define — `sys_user`'s
@@ -207,7 +217,7 @@
  * is a key whose `COMPOSE_KEY_DISPOSITIONS` disposition puts it inside an
  * ADR-0130 D4 entry's assembled body: `objects` (#19064) plus `views`,
  * `pages`, `actions`, `apps`, `dashboards` and `flows` (#19349),
- * `positions` and `permissions`, and `reports`.
+ * `positions` and `permissions`, `reports`, and `books`.
  *
  * ⚠️ `apps` reads BOTH that carrier and `navigationContributions`, and they
  * answer different questions — what a sibling DECLARES, and what this package
@@ -228,6 +238,7 @@ import { AUDIENCE_ANCHOR_POSITIONS, expandViewContainer } from '@objectstack/spe
 import { APPROVAL_NODE_TYPE, flowScreenFieldOptionKey } from '@objectstack/spec/automation';
 import {
   FLOW_TERMINAL_MESSAGE_KEYS,
+  deriveImplicitPackageBook,
   PAGE_TABS_COMPONENT_TYPE,
   hasPlatformObjectPrefix,
   isPlatformProvidedObjectName,
@@ -268,6 +279,11 @@ const END_NODE_TYPE = 'end';
  */
 const PAGE_ACCORDION_COMPONENT_TYPE = 'page:accordion';
 const REFUSED_OUTCOME = 'refused';
+/**
+ * The one group key of the implicit per-package book, read off the spec's own
+ * synthesis so the two cannot drift (#26690).
+ */
+const IMPLICIT_BOOK_GROUP_KEY = deriveImplicitPackageBook('_').groups[0].key;
 
 export const TRANSLATION_TARGET_UNKNOWN = 'translation-target-unknown';
 export const TRANSLATION_OPTION_KEY_UNKNOWN = 'translation-option-key-unknown';
@@ -516,6 +532,11 @@ interface Universe {
   permissionSets: Set<string>;
   /** Report names (`Report.name`) a `reports.<name>` key may name. */
   reports: Set<string>;
+  /**
+   * Book name (`Book.name`) → the group `key`s a `books.<name>.groups.<key>`
+   * key may name; the implicit per-package book included (#26690).
+   */
+  books: Map<string, Set<string>>;
   /**
    * Page name (`Page.name`) → every component id the page ADDRESSES
    * (`pages.<page>.components.<id>`), with that component's type and, for a
@@ -1692,6 +1713,30 @@ function buildUniverse(stack: AnyRec): Universe {
   // `reports` carries disposition `concat` too, so a sibling's reports resolve.
   const reports = namesOf('reports');
 
+  // ── Doc books: the names `books.<name>` may name, and their group keys (#26690) ──
+  //
+  // `books` carries disposition `concat` too. A package's implicit book (named
+  // by its id, one `all` group) is served wherever no book of that name is
+  // authored, so its keys resolve as well.
+  const books = new Map<string, Set<string>>();
+  for (const record of [...recordsOf(stack.books), ...artifactProvidedRecords(stack, 'books')]) {
+    const name = strName(record.name);
+    if (!name) continue;
+    const keys = books.get(name) ?? new Set<string>();
+    for (const group of Array.isArray(record.groups) ? record.groups : []) {
+      const key = isRec(group) ? strName(group.key) : undefined;
+      if (key) keys.add(key);
+    }
+    books.set(name, keys);
+  }
+  const packageIds = [
+    isRec(stack.manifest) ? strName(stack.manifest.id) : undefined,
+    ...packagesOf(stack).map((entry) => (isRec(entry.manifest) ? strName(entry.manifest.id) : undefined)),
+  ];
+  for (const id of packageIds) {
+    if (id && !books.has(id)) books.set(id, new Set([IMPLICIT_BOOK_GROUP_KEY]));
+  }
+
   return {
     objects,
     extended,
@@ -1706,6 +1751,7 @@ function buildUniverse(stack: AnyRec): Universe {
     positions,
     permissionSets,
     reports,
+    books,
     pages,
   };
 }
@@ -2178,6 +2224,35 @@ export function validateTranslationReferences(stack: AnyRec): TranslationRefFind
         }
       }
 
+      // ── books.<name>[.groups.<key>] (#26690) ──────────────────────────
+      for (const [bookName, rawBook] of Object.entries(asRecord(rawData.books))) {
+        const bookPath = `${base}.books.${bookName}`;
+        const groupKeys = universe.books.get(bookName);
+        if (!groupKeys) {
+          orphan(
+            `${inLocale} · book "${bookName}"`,
+            bookPath,
+            `Translations are keyed to book "${bookName}", which no package in this stack declares, ` +
+              `so nothing resolves this key and the book keeps its source-locale title.` +
+              suggest(bookName, universe.books.keys()),
+            "Match the key to a book's `name`, or drop it.",
+          );
+          continue;
+        }
+        if (!isRec(rawBook)) continue;
+        for (const groupKey of Object.keys(asRecord(rawBook.groups))) {
+          if (groupKeys.has(groupKey)) continue;
+          orphan(
+            `${inLocale} · book "${bookName}" · group "${groupKey}"`,
+            `${bookPath}.groups.${groupKey}`,
+            `Translations are keyed to group "${groupKey}" of book "${bookName}", which declares no group ` +
+              `with that \`key\`, so the docs sidebar keeps the section's source-locale title.` +
+              suggest(groupKey, groupKeys),
+            "Match the key to a group's `key` (not its label), or drop it.",
+          );
+        }
+      }
+
       // ── flows.<name>[.screens.<node_id>[.fields.<field_name>]] ────────
       for (const [flowName, rawFlow] of Object.entries(asRecord(rawData.flows))) {
         const flowPath = `${base}.flows.${flowName}`;
diff --git a/packages/rest/src/meta-item-read-gate.ts b/packages/rest/src/meta-item-read-gate.ts
index d261d27030f..0bd3fcaa294 100644
--- a/packages/rest/src/meta-item-read-gate.ts
+++ b/packages/rest/src/meta-item-read-gate.ts
@@ -3926,7 +3926,8 @@ export function createMetaLayeredAnswer(
 // ── THE book tree ─────────────────────────────────────────────────────────────
 
 /** Everything {@link createMetaBookTreeAnswer} reads, supplied by the transport. */
-export interface MetaBookTreeSources extends MetaReadGateAudienceSources {
+export interface MetaBookTreeSources
+    extends MetaReadGateAudienceSources, Pick<MetaListTranslationSources, 'resolveI18nService'> {
     /**
      * One list read of `type` for the tree, env-wide, scoped to `packageId` when
      * the request names one (`?package=`, ADR-0048). A THROW propagates: an
@@ -3972,7 +3973,13 @@ export type MetaBookTreeAnswer =
  *     (fail closed when holdings cannot be resolved, ADR-0049).
  *  4. Every doc (the same package scope), collapsed to the request's locale and
  *     reduced to the header the tree renders.
- *  5. The tree, its ENTRIES narrowed by each doc's effective audience, so a
+ *  5. [#26690] The book itself in the request's locale — its `label` and each
+ *     group's `label` from the bundle's `books` group — through
+ *     {@link translateMetaDocument}, the table every `/meta/book` read
+ *     translates through, so the tree and the list the console's sidebar
+ *     draws cannot disagree. Translated AFTER the gate: the audience is
+ *     judged on the book as stored, and a label is not an access input.
+ *  6. The tree, its ENTRIES narrowed by each doc's effective audience, so a
  *     reader never sees an entry that would refuse them on fetch.
  */
 export async function createMetaBookTreeAnswer(
@@ -4003,5 +4010,7 @@ export async function createMetaBookTreeAnswer(
             tags: d.tags,
             packageId: d._packageId,
         }));
-    return { ok: true, tree: audience.readableTree(book, docs) };
+    // A book has no packaged base (`PACKAGED_BASE_ACCESSORS`), so the protocol is never asked for.
+    const served = await translateMetaDocument({ ...sources, resolveProtocol: async () => undefined }, 'book', book);
+    return { ok: true, tree: audience.readableTree(served as typeof book, docs) };
 }
diff --git a/packages/rest/src/meta-item-read-routes.ts b/packages/rest/src/meta-item-read-routes.ts
index 5f398b04809..ff28f31f741 100644
--- a/packages/rest/src/meta-item-read-routes.ts
+++ b/packages/rest/src/meta-item-read-routes.ts
@@ -389,6 +389,8 @@ export function registerMetaItemReadRoutes(this: MetaItemReadRouteHost, scope: M
                                 return prot.getMetaItems(request);
                             },
                             requestLocale: (i18n) => this.extractLocale(req, i18n),
+                            // [#26690] The book's own labels, from the bundle's `books` group.
+                            resolveI18nService: () => this.resolveI18nService(environmentId, req),
                         },
                         { name: req.params.name, packageId },
                     );
diff --git a/packages/runtime/src/domains/meta-list-projection-parity.test.ts b/packages/runtime/src/domains/meta-list-projection-parity.test.ts
index 1a8af3f249f..d9b721d3667 100644
--- a/packages/runtime/src/domains/meta-list-projection-parity.test.ts
+++ b/packages/runtime/src/domains/meta-list-projection-parity.test.ts
@@ -226,6 +226,8 @@ const BUNDLE: Record<string, any> = {
         positions: { sales_rep: { label: '销售代表' } },
         permissions: { crm_sales_user: { label: '销售用户' } },
         reports: { opps_by_stage: { label: '各阶段商机' } },
+        // [#26690] A doc book's title and section titles.
+        books: { help_center: { label: '帮助中心', groups: { start: { label: '开始' } } } },
     },
 };
 
@@ -688,6 +690,12 @@ describe('[#20320] row A: the reference moves — each probe changes RestServer\
         // [#25788] So is a report — the list the console's report index reads.
         const zhReports = await read('/meta/reports', {}, { 'accept-language': 'zh-CN' });
         expect(zhReports.items?.find((r: any) => r.name === 'opps_by_stage')?.label).toBe('各阶段商机');
+        // [#26690] So is a doc book — the list the console's docs sidebar is built from.
+        const zhBooks = await read('/meta/books', {}, { 'accept-language': 'zh-CN' });
+        const zhHelp = zhBooks.items?.find((b: any) => b.name === 'help_center');
+        expect([zhHelp?.label, zhHelp?.groups?.map((g: any) => g.label)]).toEqual(['帮助中心', ['开始']]);
+        const enHelp = (await read('/meta/book', {}, { 'accept-language': 'en-US' })).items?.find((b: any) => b.name === 'help_center');
+        expect([enHelp?.label, enHelp?.groups?.map((g: any) => g.label)]).toEqual(['Help Centre', ['Start']]);
     });
 });
 
@@ -1005,6 +1013,7 @@ describe('[#20408] the item read: the reference moves — each probe changes Res
         expect((await read('/meta/position/sales_rep', {}, { 'accept-language': 'zh-CN' })).item?.label).toBe('销售代表');
         expect((await read('/meta/permission/crm_sales_user', {}, { 'accept-language': 'zh-CN' })).item?.label).toBe('销售用户');
         expect((await read('/meta/report/opps_by_stage', {}, { 'accept-language': 'zh-CN' })).item?.label).toBe('各阶段商机');
+        expect((await read('/meta/book/help_center', {}, { 'accept-language': 'zh-CN' })).item?.groups?.[0]?.label).toBe('开始');
         const zhDoc = await read('/meta/doc/crm_intro', { locale: 'zh-CN' });
         expect(zhDoc.item?.label).toBe('入门');
         expect(text(zhDoc)).not.toContain('"translations"');
@@ -1236,6 +1245,20 @@ describe('[#20408] GET /meta/book/:name/tree: the dispatcher serves the route Re
         expect({ status: member.status, code: member.code }).toEqual({ status: 403, code: 'PERMISSION_DENIED' });
         expect(text(await bootRest('holder').read('/meta/book/admin_guide/tree'))).toContain('crm_admin_runbook');
     });
+
+    it('[#26690] the tree serves the request locale: a zh-CN reader gets the book title and section titles, an en-US reader the authored ones', async () => {
+        for (const [transport, boot] of [['rest', bootRest], ['dispatcher', bootDispatcher]] as const) {
+            // The authored group only: the synthetic `uncategorized` orphan group is not a book key.
+            const authored = (tree: any) => tree?.groups?.filter((g: any) => g.key === 'start');
+            const zh = served(await boot('holder').read('/meta/book/help_center/tree', {}, { 'accept-language': 'zh-CN' }), transport) as any;
+            expect([zh?.label, authored(zh)?.map((g: any) => g.label)], transport).toEqual(['帮助中心', ['开始']]);
+            // The entry beneath keeps its own `doc.translations` localization.
+            expect(authored(zh)?.[0]?.entries?.map((e: any) => e.label), transport).toEqual(['入门']);
+            const en = served(await boot('holder').read('/meta/book/help_center/tree', {}, { 'accept-language': 'en-US' }), transport) as any;
+            expect([en?.label, authored(en)?.map((g: any) => g.label)], transport).toEqual(['Help Centre', ['Start']]);
+            expect(authored(en)?.[0]?.entries?.map((e: any) => e.label), transport).toEqual(['Getting started']);
+        }
+    });
 });
 
 describe('[#20408] an undetermined field visibility (ADR-0106 D6 tier 2) serves the object schema `private, no-store` on both transports', () => {
diff --git a/packages/runtime/src/domains/meta.ts b/packages/runtime/src/domains/meta.ts
index d7f49d46d24..364cbbee92d 100644
--- a/packages/runtime/src/domains/meta.ts
+++ b/packages/runtime/src/domains/meta.ts
@@ -1141,9 +1141,12 @@ export async function handleMetadataRequest(deps: DomainHandlerDeps, path: strin
             const answer = await createMetaBookTreeAnswer(
                 {
                     ...metaItemReadGateSources(deps, _context, protocol),
+                    // [#26690] The translation I/O the item and list reads use: the
+                    // tree translates its book (the bundle's `books` group) the same way.
+                    ...metaTranslationSources(deps, _context, protocol, (i18n) =>
+                        metaRequestLocale({ headers: _context.request?.headers, query }, i18n)),
                     listTreeInput: (type, scopedPackageId) =>
                         Promise.resolve(protocol.getMetaItems!({ type, ...(scopedPackageId ? { packageId: scopedPackageId } : {}) })),
-                    requestLocale: (i18n) => metaRequestLocale({ headers: _context.request?.headers, query }, i18n),
                 },
                 { name: decodeMetaNameSegment(parts[1]), packageId },
             );
diff --git a/packages/spec/api-surface/system.json b/packages/spec/api-surface/system.json
index 982d40c2cbe..c102c7586d5 100644
--- a/packages/spec/api-surface/system.json
+++ b/packages/spec/api-surface/system.json
@@ -73,9 +73,11 @@
     "BookAudience (type)",
     "BookAudienceSchema (const)",
     "BookGroup (type)",
+    "BookGroupLike (interface)",
     "BookGroupSchema (const)",
     "BookInclude (type)",
     "BookIncludeSchema (const)",
+    "BookLike (interface)",
     "BookNode (type)",
     "BookNodeSchema (const)",
     "BookSchema (const)",
@@ -891,6 +893,7 @@
     "toLocaleDescriptors (function)",
     "translateAction (function)",
     "translateApp (function)",
+    "translateBook (function)",
     "translateDashboard (function)",
     "translateDataset (function)",
     "translateFlow (function)",
diff --git a/packages/spec/authorable-surface/system.json b/packages/spec/authorable-surface/system.json
index 1be1a850ce8..17dfed4a5bc 100644
--- a/packages/spec/authorable-surface/system.json
+++ b/packages/spec/authorable-surface/system.json
@@ -903,6 +903,7 @@
     "system/Plan:priceMonthly",
     "system/Plan:priceYearly",
     "system/PlatformTranslationData:apps",
+    "system/PlatformTranslationData:books",
     "system/PlatformTranslationData:dashboards",
     "system/PlatformTranslationData:datasets",
     "system/PlatformTranslationData:flows",
@@ -1031,6 +1032,7 @@
     "system/SecurityEventCorrelation:linkEncryptionToAudit",
     "system/SecurityEventCorrelation:linkMaskingToAudit",
     "system/ServedTranslationData:apps",
+    "system/ServedTranslationData:books",
     "system/ServedTranslationData:dashboards",
     "system/ServedTranslationData:datasets",
     "system/ServedTranslationData:flows",
@@ -1333,6 +1335,7 @@
     "system/TranslationCoverageResult:totalKeys",
     "system/TranslationCoverageResult:translatedKeys",
     "system/TranslationData:apps",
+    "system/TranslationData:books",
     "system/TranslationData:dashboards",
     "system/TranslationData:datasets",
     "system/TranslationData:flows",
@@ -1361,6 +1364,7 @@
     "system/TranslationItem:_packageVersion",
     "system/TranslationItem:_provenance",
     "system/TranslationItem:apps",
+    "system/TranslationItem:books",
     "system/TranslationItem:dashboards",
     "system/TranslationItem:datasets",
     "system/TranslationItem:flows",
diff --git a/packages/spec/export-origins/system.json b/packages/spec/export-origins/system.json
index d5a7c120bc8..9b4f706b011 100644
--- a/packages/spec/export-origins/system.json
+++ b/packages/spec/export-origins/system.json
@@ -72,9 +72,11 @@
     "BookAudience": "src/system/book.zod.ts#BookAudience (type)",
     "BookAudienceSchema": "src/system/book.zod.ts#BookAudienceSchema (const)",
     "BookGroup": "src/system/book.zod.ts#BookGroup (type)",
+    "BookGroupLike": "src/system/i18n-resolver.ts#BookGroupLike (interface)",
     "BookGroupSchema": "src/system/book.zod.ts#BookGroupSchema (const)",
     "BookInclude": "src/system/book.zod.ts#BookInclude (type)",
     "BookIncludeSchema": "src/system/book.zod.ts#BookIncludeSchema (const)",
+    "BookLike": "src/system/i18n-resolver.ts#BookLike (interface)",
     "BookNode": "src/system/book.zod.ts#BookNode (type)",
     "BookNodeSchema": "src/system/book.zod.ts#BookNodeSchema (const)",
     "BookSchema": "src/system/book.zod.ts#BookSchema (const)",
@@ -852,6 +854,7 @@
     "toLocaleDescriptors": "src/system/i18n-resolver.ts#toLocaleDescriptors (function)",
     "translateAction": "src/system/i18n-resolver.ts#translateAction (function)",
     "translateApp": "src/system/i18n-resolver.ts#translateApp (function)",
+    "translateBook": "src/system/i18n-resolver.ts#translateBook (function)",
     "translateDashboard": "src/system/i18n-resolver.ts#translateDashboard (function)",
     "translateDataset": "src/system/i18n-resolver.ts#translateDataset (function)",
     "translateFlow": "src/system/i18n-resolver.ts#translateFlow (function)",
diff --git a/packages/spec/liveness/state-counts/translation.md b/packages/spec/liveness/state-counts/translation.md
index 3979c0c9e77..87b8234dd48 100644
--- a/packages/spec/liveness/state-counts/translation.md
+++ b/packages/spec/liveness/state-counts/translation.md
@@ -12,4 +12,4 @@ committed anywhere: `check:liveness` sums the shards when it reads them.
 
 | Type | live | exp | elsewhere | dead | planned | classified |
 |---|---|---|---|---|---|---|
-| `translation` | 38 | 0 | 0 | 0 | 0 | 38 |
+| `translation` | 41 | 0 | 0 | 0 | 0 | 41 |
diff --git a/packages/spec/liveness/translation.json b/packages/spec/liveness/translation.json
index e8e745cbf50..d4dd272ab1a 100644
--- a/packages/spec/liveness/translation.json
+++ b/packages/spec/liveness/translation.json
@@ -266,6 +266,40 @@
         }
       }
     },
+    "books": {
+      "status": "live",
+      "verifiedAt": "2026-10-27",
+      "evidence": "packages/spec/src/system/i18n-resolver.ts#translateBook (`books.<name>.label` / `.description` and `books.<name>.groups.<key>.label`, registered in METADATA_DOCUMENT_TRANSLATORS); packages/rest/src/meta-item-read-gate.ts#translateMetaList (the /meta list read that applies it per request locale); packages/rest/src/meta-item-read-gate.ts#createMetaBookTreeAnswer (the book tree translates the book it resolves through the same table)",
+      "note": "Seeded LIVE with its readers in the same change (the datasets precedent): a served `book` document is localized at the REST boundary, list and item, and `GET /meta/book` is what the console's docs portal builds its sidebar from; the tree route translates the resolved book before rendering the spine. Retires nothing: the book's inline `translations` map stays a tombstone, and its prescription now points here.",
+      "children": {
+        "label": {
+          "status": "live",
+          "verifiedAt": "2026-10-27",
+          "evidence": "packages/spec/src/system/i18n-resolver.ts#lookupBookAttr (`books.<name>.label`, read by translateBook)",
+          "note": "Overlays the served book's own `label` — the docs sidebar title."
+        },
+        "description": {
+          "status": "live",
+          "verifiedAt": "2026-10-27",
+          "evidence": "packages/spec/src/system/i18n-resolver.ts#lookupBookAttr (`books.<name>.description`, read by translateBook)",
+          "note": "Overlays the served book's own `description`."
+        },
+        "groups": {
+          "status": "live",
+          "verifiedAt": "2026-10-27",
+          "evidence": "packages/spec/src/system/i18n-resolver.ts#lookupBookGroupLabel (`books.<name>.groups.<key>.label`, read by translateBook)",
+          "note": "Overlays each group's `label`, addressed by the group's stable `key` — the sidebar's section titles.",
+          "children": {
+            "label": {
+              "status": "live",
+              "verifiedAt": "2026-10-27",
+              "evidence": "packages/spec/src/system/i18n-resolver.ts#lookupBookGroupLabel (`books.<name>.groups.<key>.label`, read by translateBook)",
+              "note": "The group's section title in the docs sidebar and in the resolved tree's `groups[].label`."
+            }
+          }
+        }
+      }
+    },
     "metadataForms": {
       "status": "live",
       "verifiedAt": "2026-08-28",
diff --git a/packages/spec/src/system/book.zod.ts b/packages/spec/src/system/book.zod.ts
index 16f47667ea0..d3001a5dbdc 100644
--- a/packages/spec/src/system/book.zod.ts
+++ b/packages/spec/src/system/book.zod.ts
@@ -59,10 +59,17 @@ export type BookInclude = string | { tag: string };
  * The trap here was PROXIMITY, not plausibility. `doc.translations` — two files
  * over, the same shape, the same name — is read on every path that renders a
  * doc. So a book's own map reads as the same feature switched on one level up.
- * It never was: the tree endpoint and the portal render `label` / `description`
- * verbatim, and the generic bundle translator covers view / action / object /
- * app / dashboard / page only (`i18n-resolver.ts`). A localized book was parsed,
- * stored, round-tripped — and rendered in the authoring locale to every reader.
+ * It never was: no resolver read a book's own map, and the tree endpoint and
+ * the portal rendered `label` / `description` verbatim. A localized book was
+ * parsed, stored, round-tripped — and rendered in the authoring locale to every
+ * reader.
+ *
+ * The spine's strings now localize through the one path the rest of the
+ * platform uses: the translation bundle's `books` group
+ * (`books.<name>.label` / `.description`, `books.<name>.groups.<key>.label`),
+ * overlaid by `translateBook` (`i18n-resolver.ts`) on `GET /meta/book` and on
+ * the tree. So the prescription is a move, not only a delete — and the inline
+ * map stays retired, because a second translation path is the trap.
  *
  * Shared by both levels deliberately: an author who localized one almost
  * certainly localized the other, and splitting the wording would make the second
@@ -70,12 +77,12 @@ export type BookInclude = string | { tag: string };
  */
 const BOOK_TRANSLATIONS_RETIRED =
   'Inline `translations` on a book (and on a book group) was removed in @objectstack/spec '
-  + '17.0.0 (ADR-0049) — no resolver ever read it. The book tree endpoint and the '
-  + 'docs portal render `label` / `description` verbatim in every locale, so a localized '
-  + 'book shipped its authoring-locale strings to every reader. Delete the key. NOTE the '
-  + 'near neighbour that DOES work: `doc.translations` is live and read on every doc render '
-  + 'path — localize the docs themselves, and the portal picks the reader\'s locale up from '
-  + 'there. '
+  + '17.0.0 (ADR-0049) — no resolver ever read it, so a localized book shipped its '
+  + 'authoring-locale strings to every reader. Delete the key and move each locale\'s strings '
+  + 'into the translation bundle: `books.<book>.label` / `.description` and '
+  + '`books.<book>.groups.<group_key>.label` (`os i18n extract` seeds them). The book list, '
+  + 'the book tree and the docs sidebar resolve those per reader locale. A doc\'s own title '
+  + 'and body still localize through `doc.translations`. '
   + 'This edit left `os migrate meta` with the protocol-16.x conversions: apply it by hand, or run the tool on a release that still carries the conversion — https://objectstack.ai/docs/protocol-upgrade.';
 
 export const BookGroupSchema = lazySchema(() =>
@@ -84,7 +91,7 @@ export const BookGroupSchema = lazySchema(() =>
       .string()
       .regex(/^[a-z][a-z0-9_]*$/, 'group key must be lowercase snake_case')
       .describe('Stable group key (used by overrides, deep links, explicit `doc.group`)'),
-    label: z.string().describe('Section title — first-class, i18n-homed'),
+    label: z.string().describe('Section title — first-class, i18n-homed: translated by `books.<book>.groups.<key>.label` in the translation bundle'),
     // TOMBSTONE, not a deletion: `BookGroupSchema` is a plain `z.object` with
     // no `.strict()`, so a bare delete would have zod silently STRIP the key —
     // replacing one silent no-op with another. `retiredKey` types it `never`
@@ -150,7 +157,7 @@ export const BookSchema = lazySchema(() =>
       .string()
       .regex(/^[a-z][a-z0-9_]*$/, 'name must be lowercase snake_case')
       .describe('Book name (namespace prefix recommended, like every metadata name)'),
-    label: z.string().optional().describe('Display title'),
+    label: z.string().optional().describe('Display title — translated by `books.<book>.label` in the translation bundle'),
     description: z.string().optional(),
     // `translations` removed in 17.0.0 (#4667) — see BOOK_TRANSLATIONS_RETIRED.
     slug: z.string().optional().describe('Portal URL segment; defaults to name sans prefix'),
diff --git a/packages/spec/src/system/i18n-book.test.ts b/packages/spec/src/system/i18n-book.test.ts
new file mode 100644
index 00000000000..114ff56ddc8
--- /dev/null
+++ b/packages/spec/src/system/i18n-book.test.ts
@@ -0,0 +1,126 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// `books.<name>.…` — a doc book's translation group, and the translator that
+// overlays it on the served `book` document.
+//
+// Before this group existed a doc's body followed the reader's locale while
+// the frame around it did not: the book title and every sidebar section title
+// shipped in the authoring language, because the book's own inline map had
+// been retired (no resolver ever read it) and the bundle had no key for it.
+
+import { describe, it, expect } from 'vitest';
+import {
+  TranslationDataSchema,
+  TranslationItemSchema,
+  PlatformTranslationDataSchema,
+  type TranslationBundle,
+} from './translation.zod';
+import { translateMetadataDocument, translateBook, TRANSLATABLE_METADATA_TYPES } from './i18n-resolver';
+// The fixture is PARSED by the authoring schema rather than hand-shaped, so a
+// translator reading a member the schema does not carry cannot pass here.
+import { BookSchema, resolveBookTree } from './book.zod';
+
+const book = () => BookSchema.parse({
+  name: 'crm_manual',
+  label: 'User Manual',
+  description: 'Everything a sales rep needs',
+  groups: [
+    { key: 'start', label: 'Getting started', include: 'crm_start_*' },
+    { key: 'pipeline', label: 'Pipeline', include: 'crm_pipe_*' },
+  ],
+}) as any;
+
+const bundle: TranslationBundle = {
+  'zh-CN': {
+    books: {
+      crm_manual: {
+        label: '使用手册',
+        description: '销售代表需要的一切',
+        groups: { start: { label: '快速上手' }, pipeline: { label: '销售管道' } },
+      },
+    },
+  },
+};
+
+describe('TranslationDataSchema — books', () => {
+  it('opens at all three doors — bundle entry, platform bundle and registered item', () => {
+    expect(TranslationDataSchema.safeParse(bundle['zh-CN']).success).toBe(true);
+    expect(PlatformTranslationDataSchema.safeParse(bundle['zh-CN']).success).toBe(true);
+    expect(TranslationItemSchema.safeParse({ locale: 'zh-CN', ...bundle['zh-CN'] }).success).toBe(true);
+  });
+
+  it('closes the face at the book\'s `label` / `description` and each group\'s `label`', () => {
+    for (const entry of [{ crm_manual: { icon: 'book' } }, { crm_manual: { groups: { start: { order: 1 } } } }]) {
+      const result = TranslationDataSchema.safeParse({ books: entry });
+      expect(result.success).toBe(false);
+      expect(result.error!.issues.map((i) => i.code)).toContain('unrecognized_keys');
+    }
+  });
+
+  it('renames the singular and the book\'s own aliases onto the declared keys', () => {
+    expect(() => TranslationDataSchema.parse({ book: { b: { label: 'x' } } })).toThrow(/books/);
+    expect(() => TranslationDataSchema.parse({ books: { b: { title: 'x' } } })).toThrow(/label/);
+    expect(() => TranslationDataSchema.parse({ books: { b: { sections: {} } } })).toThrow(/groups/);
+    expect(() => TranslationDataSchema.parse({ books: { b: { groups: { g: { title: 'x' } } } } })).toThrow(/label/);
+  });
+
+  it('the inline map on the book stays retired, and its prescription names the bundle group', () => {
+    const result = BookSchema.safeParse({ ...book(), translations: { 'zh-CN': { label: '使用手册' } } });
+    expect(result.success).toBe(false);
+    expect(JSON.stringify(result.error!.issues)).toContain('books.<book>.groups.<group_key>.label');
+  });
+});
+
+describe('translateBook', () => {
+  it('is registered in the dispatch table, so the REST boundary follows', () => {
+    // `TRANSLATABLE_METADATA_TYPES` is DERIVED from the dispatch table and
+    // `@objectstack/rest` reads it: `GET /meta/book` (list and item) is
+    // localized, and the tree route translates through the same table.
+    expect(TRANSLATABLE_METADATA_TYPES.has('book')).toBe(true);
+  });
+
+  it('a zh-CN reader gets the book title, description and every section title', () => {
+    const out = translateMetadataDocument('book', book(), bundle, { locale: 'zh-CN' }) as any;
+    expect(out.label).toBe('使用手册');
+    expect(out.description).toBe('销售代表需要的一切');
+    expect(out.groups.map((g: any) => [g.key, g.label])).toEqual([['start', '快速上手'], ['pipeline', '销售管道']]);
+    // Identity and the membership rules ride through untouched.
+    expect(out.name).toBe('crm_manual');
+    expect(out.groups.map((g: any) => g.include)).toEqual(['crm_start_*', 'crm_pipe_*']);
+  });
+
+  it('the translated book renders a translated spine through the tree resolver', () => {
+    const docs = [{ name: 'crm_start_login', label: '登录' }, { name: 'crm_pipe_stages', label: '阶段' }];
+    const tree = resolveBookTree(translateBook(book(), bundle, { locale: 'zh-CN' }), docs);
+    expect(tree.label).toBe('使用手册');
+    expect(tree.groups.map((g) => g.label)).toEqual(['快速上手', '销售管道']);
+  });
+
+  it('an en-US reader is unchanged — the same reference, the authored strings', () => {
+    const doc = book();
+    expect(translateBook(doc, bundle, { locale: 'en-US' })).toBe(doc);
+    expect(translateBook(doc, undefined, { locale: 'zh-CN' })).toBe(doc);
+  });
+
+  it('a missing translation falls back to the authored string, per key', () => {
+    const partial: TranslationBundle = { 'zh-CN': { books: { crm_manual: { groups: { pipeline: { label: '销售管道' } } } } } };
+    const out = translateBook(book(), partial, { locale: 'zh-CN' }) as any;
+    expect(out.label).toBe('User Manual');
+    expect(out.description).toBe('Everything a sales rep needs');
+    expect(out.groups.map((g: any) => g.label)).toEqual(['Getting started', '销售管道']);
+  });
+
+  it('reads only its own book\'s entry, and applies the BCP-47 ladder and the fallback chain', () => {
+    const other = BookSchema.parse({ name: 'crm_admin_manual', label: 'Admin Manual', groups: [{ key: 'start', label: 'Start' }] }) as any;
+    expect(translateBook(other, bundle, { locale: 'zh-CN' })).toBe(other);
+    expect((translateBook(book(), bundle, { locale: 'zh' }) as any).label).toBe('使用手册');
+    expect((translateBook(book(), bundle, { locale: 'fr-FR', fallbackChain: ['zh-CN'] }) as any).label).toBe('使用手册');
+  });
+
+  it('does not mutate the input', () => {
+    const doc = book();
+    const before = JSON.parse(JSON.stringify(doc));
+    translateBook(doc, bundle, { locale: 'zh-CN' });
+    expect(doc).toEqual(before);
+  });
+});
diff --git a/packages/spec/src/system/i18n-resolver.ts b/packages/spec/src/system/i18n-resolver.ts
index 01a0ea1bd25..12c4841edec 100644
--- a/packages/spec/src/system/i18n-resolver.ts
+++ b/packages/spec/src/system/i18n-resolver.ts
@@ -1518,6 +1518,10 @@ const METADATA_DOCUMENT_TRANSLATORS: Record<
   // The console's report index and report page both read `GET /meta/report`,
   // so registering here is the whole wiring.
   report: translateReport,
+  // A doc book's title and section titles: the console's docs portal builds
+  // its sidebar from `GET /meta/book`, and the tree route translates the book
+  // it resolves through this same table (`createMetaBookTreeAnswer`).
+  book: translateBook,
 };
 
 /**
@@ -2232,6 +2236,98 @@ export function translatePermissionSet<T extends SecurityCatalogItemLike>(
   return translateSecurityCatalogItem('permissions', doc, bundle, opts);
 }
 
+// ────────────────────────────────────────────────────────────────────────────
+// Doc-book resolver (label + description + group labels)
+// ────────────────────────────────────────────────────────────────────────────
+
+/** Minimal book-group shape consumed by {@link translateBook}. */
+export interface BookGroupLike {
+  key?: string;
+  label?: string;
+  [key: string]: any;
+}
+
+/** Minimal doc-book shape consumed by {@link translateBook}. */
+export interface BookLike {
+  name: string;
+  label?: string;
+  description?: string;
+  groups?: BookGroupLike[];
+  [key: string]: any;
+}
+
+function lookupBookAttr(
+  bundle: TranslationBundle | undefined,
+  bookName: string,
+  attr: 'label' | 'description',
+  opts?: ResolveOptions,
+): string | undefined {
+  if (!bundle) return undefined;
+  for (const code of localeChain(opts)) {
+    const candidate = pickData(bundle, code)?.books?.[bookName]?.[attr];
+    if (typeof candidate === 'string' && candidate.length > 0) return candidate;
+  }
+  return undefined;
+}
+
+function lookupBookGroupLabel(
+  bundle: TranslationBundle | undefined,
+  bookName: string,
+  groupKey: string,
+  opts?: ResolveOptions,
+): string | undefined {
+  if (!bundle) return undefined;
+  for (const code of localeChain(opts)) {
+    const candidate = pickData(bundle, code)?.books?.[bookName]?.groups?.[groupKey]?.label;
+    if (typeof candidate === 'string' && candidate.length > 0) return candidate;
+  }
+  return undefined;
+}
+
+/**
+ * Apply the active locale to a doc-book metadata document — its `label` and
+ * `description` against `books.<name>.label` / `.description`, and each
+ * group's `label` against `books.<name>.groups.<key>.label`, addressed by the
+ * group's stable `key` as an app's navigation node is by its `id`. The input
+ * document is not mutated; it comes back as the same reference when the
+ * bundle carries none of its keys, and a key the bundle does not carry leaves
+ * the authored string.
+ *
+ * Registered in {@link METADATA_DOCUMENT_TRANSLATORS}, so the REST boundary
+ * localizes every served book (`GET /meta/book`, list and item — the list the
+ * console's docs portal builds its sidebar from), and the book tree route
+ * (`GET /meta/book/:name/tree`) translates the book it resolves through the
+ * same table before the spine is rendered.
+ */
+export function translateBook<T extends BookLike>(
+  doc: T,
+  bundle: TranslationBundle | undefined,
+  opts?: ResolveOptions,
+): T {
+  if (!doc || typeof doc !== 'object') return doc;
+  const bookName = doc.name;
+  if (typeof bookName !== 'string' || bookName.length === 0 || !bundle) return doc;
+  const label = lookupBookAttr(bundle, bookName, 'label', opts);
+  const description = lookupBookAttr(bundle, bookName, 'description', opts);
+  let groupsMoved = false;
+  const groups = Array.isArray(doc.groups)
+    ? doc.groups.map((group) => {
+      if (!group || typeof group !== 'object' || typeof group.key !== 'string') return group;
+      const translated = lookupBookGroupLabel(bundle, bookName, group.key, opts);
+      if (translated === undefined) return group;
+      groupsMoved = true;
+      return { ...group, label: translated };
+    })
+    : doc.groups;
+  if (label === undefined && description === undefined && !groupsMoved) return doc;
+  return {
+    ...doc,
+    ...(label !== undefined ? { label } : {}),
+    ...(description !== undefined ? { description } : {}),
+    ...(groupsMoved ? { groups } : {}),
+  };
+}
+
 // ────────────────────────────────────────────────────────────────────────────
 // Report metadata resolver (label + description)
 // ────────────────────────────────────────────────────────────────────────────
diff --git a/packages/spec/src/system/translation.zod.ts b/packages/spec/src/system/translation.zod.ts
index 196f4801ff7..33196290c53 100644
--- a/packages/spec/src/system/translation.zod.ts
+++ b/packages/spec/src/system/translation.zod.ts
@@ -744,10 +744,10 @@ const PER_APP_SETTINGS_PLATFORM_ONLY =
   + '(`@objectstack/service-settings`\'s `settingsBuiltinTranslations`, typed '
   + '`PlatformTranslationData`); a key it does not translate falls back to the manifest\'s own '
   + 'literal, so correct it there rather than filling the gap from an application. For an '
-  + 'application\'s own copy use the fourteen groups this bundle does declare, in the order it '
+  + 'application\'s own copy use the fifteen groups this bundle does declare, in the order it '
   + "declares them — 'objects', 'picklists', 'apps', 'messages', 'globalActions', 'dashboards', "
-  + "'datasets', 'reports', 'pages', 'flows', 'positions', 'permissions', 'metadataForms', "
-  + "'settingsCommon'. "
+  + "'datasets', 'reports', 'pages', 'flows', 'positions', 'permissions', 'books', "
+  + "'metadataForms', 'settingsCommon'. "
   + "Note the last one: 'settingsCommon' IS "
   + 'on this face, so the Settings UI shell strings an application may translate (the source '
   + 'badges, under `settingsCommon.sourceLabels`) are NOT what is being refused here — only the '
@@ -789,7 +789,7 @@ const ITEM_SETTINGS_PLATFORM_ONLY =
   + '(`@objectstack/service-settings`\'s `settingsBuiltinTranslations`, typed '
   + '`PlatformTranslationData`); a key it does not translate falls back to the manifest\'s own '
   + 'literal, so correct it there rather than overriding it from an application. For an '
-  + 'application\'s own copy use the groups this item does declare — the same fourteen a per-app '
+  + 'application\'s own copy use the groups this item does declare — the same fifteen a per-app '
   + "bundle declares, 'settingsCommon' among them: the Settings UI shell strings an application "
   + 'may translate (the source badges, under `settingsCommon.sourceLabels`) are NOT what is being '
   + "refused here — only the per-namespace manifest copy under 'settings' is. "
@@ -828,12 +828,12 @@ const ITEM_TRANSLATION_KEY_GUIDANCE: Record<string, string> = {
  */
 /**
  * The translation groups an APPLICATION may author, as a shape rather than a
- * schema. Fourteen groups — the fifteenth, `settings`, is platform-only and lives in
+ * schema. Fifteen groups — the sixteenth, `settings`, is platform-only and lives in
  * {@link platformSettingsShape}.
  *
  * Three schemas need exactly these keys: {@link TranslationDataSchema} (one
- * entry of a per-app file-authored bundle — these fourteen and no more),
- * {@link PlatformTranslationDataSchema} (these fourteen plus `settings`) and
+ * entry of a per-app file-authored bundle — these fifteen and no more),
+ * {@link PlatformTranslationDataSchema} (these fifteen plus `settings`) and
  * {@link TranslationItemSchema} (the registered `translation` metadata type —
  * the per-app face plus `locale`, its identity keys and the ADR-0010
  * envelope; it carried the platform face's `settings` too until #19620). The item used to
@@ -1773,6 +1773,41 @@ const appTranslationDataShape = () => ({
     description: z.string().optional().describe("Translated permission-set description — overlays the set's own `description`"),
   })).optional().describe('Permission-set translations keyed by permission-set name'),
 
+  /**
+   * Doc-book translations keyed by book name (`Book.name`,
+   * `system/book.zod.ts`, the stack's `books[]`).
+   *
+   *   books.<name>.label                     → the book's own `label`
+   *   books.<name>.description               → the book's own `description`
+   *   books.<name>.groups.<group_key>.label  → a group's `label`, by the group's `key`
+   *
+   * A book is the spine of a docs table of contents, and its strings frame
+   * every page of it: the book title and each section title in the docs
+   * sidebar. The pages themselves localize through `doc.translations`; the
+   * spine localizes here, the way an app's label and navigation labels do
+   * (`apps.<name>.navigation.<id>.label`, keyed by the stable node id as a
+   * group is by its stable `key`). Overlaid by `translateBook`
+   * (`i18n-resolver.ts`) on every served `book` document (`GET /meta/book`,
+   * the list the console's docs portal draws its sidebar from) and on the
+   * book `GET /meta/book/:name/tree` resolves. The inline map a book carried
+   * before 17.0.0 stays retired: one translation path, this one.
+   */
+  books: z.record(z.string(), strictObject({
+    surface: 'this book translation',
+    history: TRANSLATION_HISTORY,
+    aliases: { name: 'label', title: 'label', sections: 'groups', chapters: 'groups', toc: 'groups' },
+  }, {
+    label: z.string().optional().describe("Translated book title — overlays the book's own `label`"),
+    description: z.string().optional().describe("Translated book description — overlays the book's own `description`"),
+    groups: z.record(z.string(), strictObject({
+      surface: 'this book group translation',
+      history: TRANSLATION_HISTORY,
+      aliases: { name: 'label', title: 'label' },
+    }, {
+      label: z.string().describe("Translated section title — overlays the group's own `label`"),
+    })).optional().describe('Group (section) translations keyed by the group `key`'),
+  })).optional().describe('Doc-book translations keyed by book name'),
+
   /**
    * Translations for **metadata-type configuration forms** — the forms
    * used by admins to author objects, fields, agents, flows, etc. in the
@@ -1987,10 +2022,10 @@ const platformSettingsShape = () => ({
  * One locale of a PER-APP translation bundle — `stack.translations`, and
  * everything {@link defineTranslationBundle} builds.
  *
- * Fourteen groups: every group the platform bundle declares EXCEPT `settings`,
+ * Fifteen groups: every group the platform bundle declares EXCEPT `settings`,
  * which is platform-only and is refused here by name with
  * {@link PER_APP_SETTINGS_PLATFORM_ONLY} as the remedy. See
- * {@link PlatformTranslationDataSchema} for the fifteen-group face and for why
+ * {@link PlatformTranslationDataSchema} for the sixteen-group face and for why
  * the two are separate namespaces.
  */
 export const TranslationDataSchema = lazySchema(() => strictObject({
@@ -2001,7 +2036,7 @@ export const TranslationDataSchema = lazySchema(() => strictObject({
   // `settings`, and an alias prescribing a key the shape rejects is a
   // suggestion the author cannot take (the `alias-integrity` audit judges
   // exactly that). Both spellings are answered by `guidance` above instead.
-  aliases: { object: 'objects', fields: 'objects', app: 'apps', page: 'pages', dashboard: 'dashboards', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions' },
+  aliases: { object: 'objects', fields: 'objects', app: 'apps', page: 'pages', dashboard: 'dashboards', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', book: 'books', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions' },
   // `locale` lives on the ITEM, not on a bundle entry (the bundle keys ARE the
   // locales). Naming it keeps the suggestion useful for an author who moved a
   // `translation` item into a bundle and left the field behind.
@@ -2011,7 +2046,7 @@ export const TranslationDataSchema = lazySchema(() => strictObject({
 export type TranslationData = z.input<typeof TranslationDataSchema>;
 
 /**
- * One locale of a PLATFORM translation bundle — the fifteen groups, `settings`
+ * One locale of a PLATFORM translation bundle — the sixteen groups, `settings`
  * included.
  *
  * The platform's own bundles are code, not authored metadata
@@ -2046,7 +2081,7 @@ export const PlatformTranslationDataSchema = lazySchema(() => strictObject({
   surface: 'this locale of the platform translation bundle',
   history: TRANSLATION_HISTORY,
   guidance: TRANSLATION_KEY_GUIDANCE,
-  aliases: { object: 'objects', fields: 'objects', app: 'apps', page: 'pages', dashboard: 'dashboards', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', setting: 'settings', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions' },
+  aliases: { object: 'objects', fields: 'objects', app: 'apps', page: 'pages', dashboard: 'dashboards', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', book: 'books', setting: 'settings', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions' },
   extraKeys: ['locale'],
 }, {
   ...appTranslationDataShape(),
@@ -2214,7 +2249,7 @@ export type TranslationConfig = z.input<typeof TranslationConfigSchema>;
  * to whoever — or whatever — authored it.
  *
  * `settings` is NOT on this door (#19620, ruling batch #210 item 2 letter B):
- * the item takes the PER-APP face, the same fourteen groups as
+ * the item takes the PER-APP face, the same fifteen groups as
  * {@link TranslationDataSchema}, and refuses `settings` (and the singular
  * `setting`) by name with {@link ITEM_SETTINGS_PLATFORM_ONLY} as the remedy.
  * The file door and the item door are two authoring surfaces for one app
@@ -2253,7 +2288,7 @@ export const TranslationItemSchema = lazySchema(() => strictObject({
   // this door no longer declares `settings`, and an alias prescribing a key
   // the shape rejects is a suggestion the author cannot take. Both spellings
   // are answered by `guidance` above instead.
-  aliases: { object: 'objects', app: 'apps', page: 'pages', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions', lang: 'locale', language: 'locale' },
+  aliases: { object: 'objects', app: 'apps', page: 'pages', dataset: 'datasets', report: 'reports', picklist: 'picklists', flow: 'flows', position: 'positions', permission: 'permissions', permissionSets: 'permissions', book: 'books', message: 'messages', strings: 'messages', labels: 'messages', actions: 'globalActions', lang: 'locale', language: 'locale' },
 }, {
   ...appTranslationDataShape(),
   locale: LocaleSchema.describe('BCP-47 locale this item translates (e.g. "zh-CN")'),
diff --git a/scripts/fixtures/i18n-walk-parity/every-group.stack.json b/scripts/fixtures/i18n-walk-parity/every-group.stack.json
index b68f688d516..061ad21d74c 100644
--- a/scripts/fixtures/i18n-walk-parity/every-group.stack.json
+++ b/scripts/fixtures/i18n-walk-parity/every-group.stack.json
@@ -178,5 +178,13 @@
   ],
   "permissions": [
     { "name": "parity_auditor", "label": "Parity Auditor", "description": "Reads parity findings" }
+  ],
+  "books": [
+    {
+      "name": "parity_handbook",
+      "label": "Parity Handbook",
+      "description": "How parity is measured",
+      "groups": [{ "key": "basics", "label": "Basics", "include": "parity_*" }]
+    }
   ]
 }
diff --git a/ui/apps/console/src/pages/docs-portal.bookLocale-26690.test.tsx b/ui/apps/console/src/pages/docs-portal.bookLocale-26690.test.tsx
new file mode 100644
index 00000000000..a2c7fd1f2fe
--- /dev/null
+++ b/ui/apps/console/src/pages/docs-portal.bookLocale-26690.test.tsx
@@ -0,0 +1,142 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * objectstack#26690 — the docs sidebar under `/apps/:appName/docs` draws a doc
+ * book's title and section titles in the reader's locale.
+ *
+ * The portal builds its sidebar from `GET /meta/book`, which the server answers
+ * in the request locale (`Accept-Language`, sent by the console's client on
+ * every request) by overlaying the bundle's `books` group through
+ * `translateBook`. This test mounts the REAL route table, layout, reader and
+ * sidebar over an adapter that answers the book list the way the server does:
+ * the authored book run through the spec's own translator with a translation
+ * bundle, for the locale the "request" carries. So the chain pinned here is
+ * bundle → served book → sidebar, with nothing hand-translated in between.
+ */
+
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, screen, cleanup, within } from '@testing-library/react';
+import { MemoryRouter, Routes, Route } from 'react-router-dom';
+// Paid at collection, not inside the adapter's clocked `getItems` call.
+import '@objectstack/spec/system';
+import type { TranslationBundle } from '@objectstack/spec/system';
+
+const { ADAPTER, REQUEST } = vi.hoisted(() => {
+  const REQUEST = { locale: 'en-US' };
+  const DOCS = [
+    { name: 'crm_start_login', label: 'Signing in', _packageId: 'com.example.crm', order: 1 },
+    { name: 'crm_pipe_stages', label: 'Stages', _packageId: 'com.example.crm', order: 2 },
+  ];
+  const BOOK = {
+    name: 'crm_manual',
+    label: 'User Manual',
+    slug: 'manual',
+    _packageId: 'com.example.crm',
+    groups: [
+      { key: 'start', label: 'Getting started', include: 'crm_start_*' },
+      { key: 'pipeline', label: 'Pipeline', include: 'crm_pipe_*' },
+    ],
+  };
+  const BUNDLE: TranslationBundle = {
+    'zh-CN': {
+      books: { crm_manual: { label: '使用手册', groups: { start: { label: '快速上手' }, pipeline: { label: '销售管道' } } } },
+    },
+  };
+  const ADAPTER = {
+    getClient: () => ({
+      meta: {
+        getItems: async (type: string) => {
+          if (type === 'doc') return DOCS;
+          if (type !== 'book') return [];
+          // What `GET /meta/book` answers: every book through the translator
+          // table, for the request's locale.
+          const { translateMetadataDocument } = await import('@objectstack/spec/system');
+          return [translateMetadataDocument('book', BOOK, BUNDLE, { locale: REQUEST.locale })];
+        },
+        getItem: async (_type: string, name: string) => ({ item: { name, content: `BODY ${name}` } }),
+      },
+    }),
+  };
+  return { ADAPTER, REQUEST };
+});
+
+vi.mock('@object-ui/app-shell', async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  useAdapter: () => ADAPTER,
+}));
+
+vi.mock('@object-ui/plugin-markdown', async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  MarkdownRenderer: ({ schema }: { schema: { content?: string } }) => (
+    <div data-testid="doc-content">{schema.content}</div>
+  ),
+  extractToc: () => [],
+}));
+
+vi.mock('@object-ui/i18n', async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  useObjectTranslation: () => ({ t: (_k: string, o?: { defaultValue?: string }) => o?.defaultValue ?? _k }),
+}));
+
+// Imported AFTER the mocks so the pages pick up the mocked modules.
+import DocsLayout from './DocsLayout';
+import DocsSlug from './DocsSlug';
+import DocPage from './DocPage';
+
+function Portal({ entry }: { entry: string }) {
+  return (
+    <MemoryRouter initialEntries={[entry]}>
+      <Routes>
+        <Route path="/apps/:appName/docs" element={<DocsLayout />}>
+          <Route path=":slug" element={<DocsSlug />} />
+          <Route path=":slug/:name" element={<DocPage />} />
+        </Route>
+      </Routes>
+    </MemoryRouter>
+  );
+}
+
+/**
+ * The book sidebar, found by its accessible name — the book's title. It renders
+ * once wide and once in the narrow disclosure, so every copy is returned.
+ */
+async function sidebars(title: string): Promise<HTMLElement[]> {
+  await screen.findByTestId('doc-content');
+  return screen.getAllByRole('navigation', { name: title });
+}
+
+afterEach(() => {
+  cleanup();
+  REQUEST.locale = 'en-US';
+});
+
+describe('objectstack#26690 — the docs sidebar serves the request locale', () => {
+  it('a zh-CN reader sees the book title and every section title in zh-CN', async () => {
+    REQUEST.locale = 'zh-CN';
+    render(<Portal entry="/apps/com.example.crm/docs/manual/crm_start_login" />);
+    const navs = await sidebars('使用手册');
+    expect(navs.length).toBeGreaterThanOrEqual(1);
+    expect(screen.queryAllByRole('navigation', { name: 'User Manual' })).toHaveLength(0);
+    for (const nav of navs) {
+      expect(within(nav).getByText('快速上手')).toBeInTheDocument();
+      expect(within(nav).getByText('销售管道')).toBeInTheDocument();
+      expect(within(nav).queryByText('Getting started')).toBeNull();
+    }
+  });
+
+  it('an en-US reader is unchanged: the authored title and section titles', async () => {
+    render(<Portal entry="/apps/com.example.crm/docs/manual/crm_start_login" />);
+    const navs = await sidebars('User Manual');
+    expect(navs.length).toBeGreaterThanOrEqual(1);
+    for (const nav of navs) {
+      expect(within(nav).getByText('Getting started')).toBeInTheDocument();
+      expect(within(nav).getByText('Pipeline')).toBeInTheDocument();
+    }
+  });
+});
diff --git a/ui/packages/app-shell/src/views/metadata-admin/i18n.ts b/ui/packages/app-shell/src/views/metadata-admin/i18n.ts
index e3a19eb3467..c596a33eeb8 100644
--- a/ui/packages/app-shell/src/views/metadata-admin/i18n.ts
+++ b/ui/packages/app-shell/src/views/metadata-admin/i18n.ts
@@ -949,6 +949,7 @@ const ENGINE_STRINGS_EN: Record<string, string> = {
   'engine.translationPreview.category.flows': 'Flows',
   'engine.translationPreview.category.positions': 'Positions',
   'engine.translationPreview.category.permissions': 'Permission Sets',
+  'engine.translationPreview.category.books': 'Doc Books',
   'engine.translationPreview.category.metadataForms': 'Metadata Forms',
   'engine.translationPreview.category.settingsCommon': 'Settings UI',
   'engine.translationPreview.coverage': 'Category coverage:',
@@ -3031,6 +3032,7 @@ const ENGINE_STRINGS_ZH: Record<string, string> = {
   'engine.translationPreview.category.flows': '流程',
   'engine.translationPreview.category.positions': '岗位',
   'engine.translationPreview.category.permissions': '权限集',
+  'engine.translationPreview.category.books': '文档书',
   'engine.translationPreview.category.metadataForms': '元数据表单',
   'engine.translationPreview.category.settingsCommon': '设置界面',
   'engine.translationPreview.coverage': '分类覆盖率：',
diff --git a/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.categories-11765.test.tsx b/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.categories-11765.test.tsx
index 26ca72b5c35..bd0bf469550 100644
--- a/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.categories-11765.test.tsx
+++ b/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.categories-11765.test.tsx
@@ -69,6 +69,11 @@ const ONE_ENTRY: Record<string, { data: Record<string, unknown>; key: string; ro
     key: 'contract_reviewer',
     row: () => keys(2),
   },
+  books: {
+    data: { crm_manual: { label: 'Manuel', groups: { start: { label: 'Démarrage' } } } },
+    key: 'crm_manual',
+    row: () => keys(2),
+  },
   metadataForms: { data: { object: { label: 'Objet' } }, key: 'object', row: () => keys(1) },
   // `settingsCommon` is one strict object, not a record of author-named nodes:
   // its members (`sourceLabels`) are what the card counts and samples.
diff --git a/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.tsx b/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.tsx
index d9fe855240a..7d6f61332ba 100644
--- a/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.tsx
+++ b/ui/packages/app-shell/src/views/metadata-admin/previews/TranslationPreview.tsx
@@ -33,6 +33,7 @@ import * as React from 'react';
 import {
   AppWindow,
   BarChart3,
+  BookOpen,
   ClipboardList,
   Database,
   FileText,
@@ -100,6 +101,7 @@ const CATEGORY_DEFS = {
   flows: { label: 'engine.translationPreview.category.flows', icon: Workflow, flat: false },
   positions: { label: 'engine.translationPreview.category.positions', icon: Users, flat: false },
   permissions: { label: 'engine.translationPreview.category.permissions', icon: ShieldCheck, flat: false },
+  books: { label: 'engine.translationPreview.category.books', icon: BookOpen, flat: false },
   metadataForms: { label: 'engine.translationPreview.category.metadataForms', icon: FileText, flat: false },
   settingsCommon: { label: 'engine.translationPreview.category.settingsCommon', icon: Settings2, flat: false },
 } satisfies Record<TranslationGroup, Omit<CategoryDef, 'key'>>;
diff --git a/ui/packages/app-shell/src/views/metadata-admin/previews/adminPreviews.i18n-10862-s2.test.tsx b/ui/packages/app-shell/src/views/metadata-admin/previews/adminPreviews.i18n-10862-s2.test.tsx
index 26a8c8eccd4..a3c4be75c71 100644
--- a/ui/packages/app-shell/src/views/metadata-admin/previews/adminPreviews.i18n-10862-s2.test.tsx
+++ b/ui/packages/app-shell/src/views/metadata-admin/previews/adminPreviews.i18n-10862-s2.test.tsx
@@ -464,6 +464,7 @@ describe('TranslationPreview reads the designer locale (objectui#10862)', () =>
         { key: 'engine.translationPreview.category.flows' },
         { key: 'engine.translationPreview.category.positions' },
         { key: 'engine.translationPreview.category.permissions' },
+        { key: 'engine.translationPreview.category.books' },
         { key: 'engine.translationPreview.category.metadataForms' },
         { key: 'engine.translationPreview.category.settingsCommon' },
         { key: 'engine.translationPreview.categoryEmpty' },
@@ -471,7 +472,7 @@ describe('TranslationPreview reads the designer locale (objectui#10862)', () =>
         { key: 'engine.translationPreview.keyCountOne', vars: { count: 1 } },
       ]);
       // The bundle's own locale is author data, not the designer's language.
-      expectAsWritten(['French', 'fr-FR', 'account', '"Un"', '2/14 (14%)']);
+      expectAsWritten(['French', 'fr-FR', 'account', '"Un"', '2/15 (13%)']);
     });
   }
 });
diff --git a/ui/packages/i18n/src/utils/spec-translations.ts b/ui/packages/i18n/src/utils/spec-translations.ts
index 0f3638a4788..b4a6ab5e78e 100644
--- a/ui/packages/i18n/src/utils/spec-translations.ts
+++ b/ui/packages/i18n/src/utils/spec-translations.ts
@@ -107,6 +107,9 @@ const SPEC_TRANSLATION_GROUPS = [
   // objectstack #25788 serves `reports` (a report's label and description are
   // translatable), copied through onto `app` the same way.
   'reports',
+  // objectstack #26690 serves `books` (a doc book's title, description and
+  // section titles are translatable), copied through onto `app` the same way.
+  'books',
 ] as const;
 
 function isPlainObject(value: unknown): value is Record<string, unknown> {