diff --git a/docs/specs/2026-06-21-add-book-form-design.md b/docs/specs/2026-06-21-add-book-form-design.md
new file mode 100644
index 0000000..f1101c6
--- /dev/null
+++ b/docs/specs/2026-06-21-add-book-form-design.md
@@ -0,0 +1,169 @@
+# Spine — Add Book Form
+
+Adds a form to create new books in `spine.org`, plus the write function
+to persist them.
+
+## Architecture
+
+```
+"Add book" link (index page)
+        │
+    GET /add
+        │
+templates/add.mustache ←── [spine-index-model] for datalist values
+        │
+    POST /add (form submission)
+        │
+    [spine-add-book] ──→ writes to spine.org
+        │
+    redirect to /index
+```
+
+## Template: `templates/add.mustache`
+
+A new Mustache template with the Pico CSS + Tabler Icons shell (matching the
+index page) and a `<form method="post" action="/add">`.
+
+**View model:**
+
+```json
+{
+  "app_title": "spine",
+  "existing_authors": ["Ursula K. Le Guin", "Frank Herbert", ...],
+  "existing_categories": ["scifi", "literary", ...]
+}
+```
+
+`existing_authors` and `existing_categories` are extracted from the current
+`spine-books` results. Empty lists are fine — the `<datalist>` just has no
+options.
+
+**Form fields:**
+
+| Field | Input type | Required | Notes |
+|---|---|---|---|
+| date_added | `<input type="date">` | no | defaults to today |
+| title | `<input type="text">` | **yes** | |
+| author | `<input type="text" list="authors">` | no | datalist from existing |
+| category | `<input type="text" list="categories">` | no | datalist from existing tags |
+| format | `<select>` | no | hardcover / ebook / audiobook / — |
+| isbn | `<input type="text">` | no | |
+| cover | `<input type="text">` | no | path relative to spine.org |
+| rec_by | `<input type="text">` | no | recommender name |
+| rec_note | `<input type="text">` | no | why it's on your radar |
+| initial_note | `<textarea>` | no | becomes first reading note |
+
+**Datalists** for author and category:
+
+```html
+<input type="text" name="author" list="authors-list" />
+<datalist id="authors-list">
+  {{#existing_authors}}<option value="{{.}}">{{/existing_authors}}
+</datalist>
+```
+
+"Add book" link on the index page (in `templates/index.mustache`) — placed in
+the `<header>` alongside the title:
+
+```html
+<a role="button" href="/add">Add book</a>
+```
+
+## Write function: `spine-add-book`
+
+```elisp
+(spine-add-book &key title author category format isbn cover
+                 rec_by rec_note date_added initial_note)
+→ t on success, signals error on failure
+```
+
+**Behavior:**
+1. Visits `spine-org-file` in a temporary buffer
+2. Inserts a new top-level headline with TODO keyword `WANT`
+3. Sets properties: `:AUTHOR:`, `:FORMAT:`, `:ISBN:`, `:COVER:`, `:REC_BY:`, `:REC_NOTE:`, `:ADDED:`, `:ID:`
+4. Sets tags if category provided
+5. Appends initial note as `- [DATE] note text` if provided
+6. Saves the buffer
+7. Returns `t`
+
+**`:ID:` generation:** short random alphanumeric, e.g. `(format "%04x-%04x" (random 65535) (random 65535))`.
+
+**`:ADDED:` date:** if `date_added` is provided, format as `[YYYY-MM-DD]`. If nil, omit the property.
+
+**Properties with nil/empty values are omitted** — only set properties that have non-empty values. This keeps the Org file clean.
+
+## Handler: `/add`
+
+```elisp
+(defservlet add text/html (path query request)
+  (let ((method (car (car request))))
+    (if (equal method "POST")
+        ;; Process form submission
+        (progn
+          (spine-add-book
+           :title (cdr (assoc "title" query))
+           :author (cdr (assoc "author" query))
+           :category (cdr (assoc "category" query))
+           :format (cdr (assoc "format" query))
+           :isbn (cdr (assoc "isbn" query))
+           :cover (cdr (assoc "cover" query))
+           :rec_by (cdr (assoc "rec_by" query))
+           :rec_note (cdr (assoc "rec_note" query))
+           :date_added (cdr (assoc "date_added" query))
+           :initial_note (cdr (assoc "initial_note" query)))
+          (httpd-redirect t "/index" 303))
+      ;; GET: show the form
+      (let ((model (spine-add-form-model (spine-books))))
+        (insert (spine-render "add.mustache" model))))))
+```
+
+`spine-add-form-model` builds the view model from `spine-books` — extracts
+unique authors and tags into flat lists.
+
+**Form GET access:** `http://localhost:8080/add`
+
+**Empty-books case:** if `spine-books` returns nil, the datalists are empty —
+the form still works, you just get no suggestions.
+
+## Error handling
+
+| Case | Behavior |
+|---|---|
+| Title missing | redirect to `/add` with error message (query param) |
+| Write fails | `spine-add-book` signals error, handler catches and shows error page |
+| `spine.org` missing | `spine-add-book` creates a new file with the `#+TODO` header and first entry |
+
+## Files changed
+
+| File | Change |
+|---|---|
+| `templates/add.mustache` | New — add-book form template |
+| `templates/index.mustache` | Modified — add "Add book" link in header |
+| `spine.el` | New functions: `spine-add-book`, `spine-add-form-model`. New handler: `/add`. |
+
+## Out of scope
+
+- Validation beyond title required
+- Edit existing book
+- Delete book
+- Cover image upload (text path only)
+- Multi-category input (single category string → single tag for now)
+- Error display on the form page (redirect on error, no inline messages)
+
+## Styling
+
+Uses Pico CSS form styles (`<input>`, `<select>`, `<textarea>`) which are
+styled automatically by the Pico framework. No additional custom CSS needed
+for the form — Pico handles layout, spacing, and theme.
+
+## Acceptance
+
+- [ ] "Add book" link visible on index page
+- [ ] `GET /add` shows form with Pico styling
+- [ ] Author and category fields have datalist suggestions from existing data
+- [ ] Submitting the form creates a new `* WANT Title` headline in `spine.org`
+- [ ] Properties are set correctly (title, author, format, ISBN, etc.)
+- [ ] Initial note appears as `- [YYYY-MM-DD] text` in the body
+- [ ] Empty optional fields are omitted (no empty properties)
+- [ ] After submission, redirect to `/index` and new book is visible
+- [ ] ERT tests still pass (model layer unchanged)