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+# Custom Section Builder API
+
+The Custom Section Builder allows users to create personalized dashboard sections by defining filter rules or manually selecting books.
+
+## Preview Collection
+
+Evaluates filter rules and returns matching items without saving the collection.
+
+**Endpoint:** `POST /api/collections/preview`
+
+**Request Body:**
+```json
+{
+  "library_id": "uuid",
+  "rules": [
+    {
+      "id": "rule1",
+      "field": "genre",
+      "operator": "equals",
+      "value": "Sci-Fi",
+      "priority": 1
+    }
+  ],
+  "manual_book_ids": ["uuid1", "uuid2"],
+  "limit": 20
+}
+```
+
+**Available Filter Fields:**
+
+| Field | Type | Operators |
+|-------|------|-----------|
+| `title` | text | contains, equals, starts_with, ends_with, regex |
+| `author` | text | contains, equals |
+| `genre` | select | equals, not_equals, in, not_in |
+| `series` | text | is_set, is_not_set, equals, contains |
+| `progress` | number | equals, not_equals, greater_than, less_than, between, is_set, is_not_set |
+| `rating` | number | equals, not_equals, greater_than, less_than, is_set, is_not_set |
+| `date_added` | date | equals, not_equals, before, after, between, last_x_days |
+| `last_read` | date | equals, before, after, between, last_x_days, is_set, is_not_set |
+| `publisher` | text | contains, equals |
+| `language` | select | equals, not_equals, in |
+| `format` | select | equals, in |
+| `tags` | text | contains, not_contains, equals |
+| `narrators` | text | contains, equals, is_set, is_not_set |
+
+**Response:**
+```json
+{
+  "items": [
+    {
+      "media_item_id": "uuid",
+      "title": "Book Title",
+      "author": "Author Name",
+      "cover_image_path": "/path/to/cover.jpg"
+    }
+  ]
+}
+```
+
+## Create Custom Section
+
+Creates a new custom collection with filter rules and/or manual book selection.
+
+**Endpoint:** `POST /api/collections`
+
+**Request Body:**
+```json
+{
+  "name": "My Custom Section",
+  "description": "My favorite Sci-Fi books",
+  "icon": "📚",
+  "color": "#9333ea",
+  "show_on_dashboard": true,
+  "auto_assign_rules": "[{\"id\":\"rule1\",\"field\":\"genre\",\"operator\":\"equals\",\"value\":\"Sci-Fi\",\"priority\":1}]",
+  "manual_book_ids": ["uuid1", "uuid2"],
+  "view_settings": {}
+}
+```
+
+**Response:** Returns the created collection object.
+
+## Frontend Implementation
+
+**Route:** `/custom-section`
+
+**Template:** `templates/custom_section.templ`
+
+**TypeScript:** `web/src/custom-section-builder.ts`
+
+Key features:
+- 14 filter fields with various operators
+- Live preview functionality
+- Search + multi-select for manual book addition
+- AND/OR logic support for combining rules
+
+## Example Use Cases
+
+### Sci-Fi Favorites
+```json
+{
+  "rules": [
+    {
+      "field": "genre",
+      "operator": "equals",
+      "value": "Sci-Fi"
+    }
+  ]
+}
+```
+
+### High Rated Books
+```json
+{
+  "rules": [
+    {
+      "field": "rating",
+      "operator": "greater_than",
+      "value": "4"
+    }
+  ]
+}
+```
+
+### Long Books (Manual Selection)
+```json
+{
+  "manual_book_ids": ["uuid1", "uuid2", "uuid3"]
+}
+```
+
+### Recently Finished Audiobooks
+```json
+{
+  "rules": [
+    {
+      "field": "format",
+      "operator": "equals",
+      "value": "Audiobook"
+    },
+    {
+      "field": "last_read",
+      "operator": "last_x_days",
+      "value": "30"
+    }
+  ]
+}
+```