diff --git a/test/spine-add-book-test.el b/test/spine-add-book-test.el
new file mode 100644
index 0000000..de05b4b
--- /dev/null
+++ b/test/spine-add-book-test.el
@@ -0,0 +1,98 @@
+;;; spine-add-book-test.el — ERT tests for spine-add-book
+
+(require 'ert)
+(require 'cl-lib)
+
+(setq spine-skip-server-start t)
+(setq spine-org-file (expand-file-name "test-add-temp.org"
+                                       (file-name-directory
+                                        (directory-file-name
+                                         (file-name-directory load-file-name)))))
+(load-file (expand-file-name "../spine.el"
+                             (file-name-directory load-file-name)))
+
+(defun spine-add-book-test--cleanup ()
+  "Remove the test Org file if it exists."
+  (ignore-errors (delete-file spine-org-file)))
+
+(ert-deftest spine-add-book-creates-headline ()
+  "spine-add-book creates a new WANT headline."
+  (spine-add-book-test--cleanup)
+  (unwind-protect
+      (progn
+        (spine-add-book :title "Test Book" :author "Author Name")
+        (let ((books (spine-books)))
+          (should books)
+          (should (= (length books) 1))
+          (let ((book (car books)))
+            (should (equal (plist-get book :title) "Test Book"))
+            (should (equal (plist-get book :status) "WANT"))
+            (should (equal (plist-get book :author) "Author Name"))
+            (should (plist-get book :id)))))
+    (spine-add-book-test--cleanup)))
+
+(ert-deftest spine-add-book-sets-all-fields ()
+  "spine-add-book sets all provided fields."
+  (spine-add-book-test--cleanup)
+  (unwind-protect
+      (progn
+        (spine-add-book
+         :title "Full Test"
+         :author "Jane Doe"
+         :format "hardcover"
+         :isbn "978-1234567890"
+         :cover "covers/test.jpg"
+         :rec_by "Friend"
+         :rec_note "You'll love this"
+         :date_added "2026-06-21"
+         :category "scifi"
+         :initial_note "Looking forward to this")
+        (let* ((books (spine-books))
+               (book (car books)))
+          (should (equal (plist-get book :title) "Full Test"))
+          (should (equal (plist-get book :author) "Jane Doe"))
+          (should (equal (plist-get book :format) "hardcover"))
+          (should (equal (plist-get book :isbn) "978-1234567890"))
+          (should (equal (plist-get book :cover) "covers/test.jpg"))
+          (should (equal (plist-get book :rec_by) "Friend"))
+          (should (equal (plist-get book :rec_note) "You'll love this"))
+          (should (equal (plist-get book :added) "[2026-06-21]"))
+          (should (equal (plist-get book :tags) '("scifi")))
+          (should (equal (plist-get book :notes)
+                         '(("2026-06-21" "Looking forward to this"))))))
+    (spine-add-book-test--cleanup)))
+
+(ert-deftest spine-add-book-omits-empty-fields ()
+  "spine-add-book does not set empty/omitted fields."
+  (spine-add-book-test--cleanup)
+  (unwind-protect
+      (progn
+        (spine-add-book :title "Minimal Book")
+        (let* ((books (spine-books))
+               (book (car books)))
+          (should (equal (plist-get book :title) "Minimal Book"))
+          (should-not (plist-get book :author))
+          (should-not (plist-get book :format))
+          (should-not (plist-get book :isbn))
+          (should-not (plist-get book :tags))))
+    (spine-add-book-test--cleanup)))
+
+(ert-deftest spine-add-book-requires-title ()
+  "spine-add-book signals error when title is missing."
+  (spine-add-book-test--cleanup)
+  (unwind-protect
+      (should-error (spine-add-book :author "No Title"))
+    (spine-add-book-test--cleanup)))
+
+(ert-deftest spine-add-book-generates-unique-ids ()
+  "spine-add-book generates different IDs for each book."
+  (spine-add-book-test--cleanup)
+  (unwind-protect
+      (progn
+        (spine-add-book :title "Book One")
+        (spine-add-book :title "Book Two")
+        (let* ((books (spine-books))
+               (id1 (plist-get (nth 0 books) :id))
+               (id2 (plist-get (nth 1 books) :id)))
+          (should (not (equal id1 id2)))))
+    (spine-add-book-test--cleanup)))