diff --git a/src/App.tsx b/src/App.tsx
index f982dbf..5fc21ea 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -6,6 +6,7 @@ import { LoginForm } from './auth/LoginForm'
 import { RequireAuth } from './auth/RequireAuth'
 import { BookList } from './books/BookList'
 import { BookDetail } from './books/BookDetail'
+import { BookForm } from './books/BookForm'
 import { secondaryButton } from './ui'
 
 export function App() {
@@ -25,6 +26,8 @@ export function AppRoutes() {
       <Route element={<RequireAuth />}>
         <Route element={<Shell />}>
           <Route path="/books" element={<BookList />} />
+          <Route path="/books/new" element={<BookForm />} />
+          <Route path="/books/:id/edit" element={<BookForm />} />
           <Route path="/books/:id" element={<BookDetail />} />
         </Route>
       </Route>
diff --git a/src/books/BookForm.test.tsx b/src/books/BookForm.test.tsx
new file mode 100644
index 0000000..f7d0e7e
--- /dev/null
+++ b/src/books/BookForm.test.tsx
@@ -0,0 +1,65 @@
+import { screen } from '@testing-library/react'
+import { describe, expect, it } from 'vitest'
+import { dune, renderApp, signedIn } from '../test/helpers'
+
+const fresh = { ...dune, current_page: 0, status: 'not_started' as const }
+
+describe('BookForm', () => {
+  it('adds a book and opens it', async () => {
+    const calls = signedIn({
+      'POST /api/books': () => [201, fresh],
+      'GET /api/books/7': () => [200, fresh],
+    })
+    const user = renderApp('/books/new')
+    await screen.findByRole('option', { name: 'Science Fiction' })
+    await user.type(screen.getByLabelText('Title'), 'Dune')
+    await user.type(screen.getByLabelText('Author'), 'Frank Herbert')
+    await user.selectOptions(screen.getByLabelText('Genre'), 'Science Fiction')
+    await user.type(screen.getByLabelText('Total pages'), '380')
+    await user.click(screen.getByRole('button', { name: 'Add book' }))
+
+    expect(await screen.findByRole('heading', { name: 'Dune', level: 1 })).toBeInTheDocument()
+    expect(calls).toContainEqual({
+      method: 'POST',
+      path: '/api/books',
+      body: { title: 'Dune', author: 'Frank Herbert', genre_id: 5, total_pages: 380, current_page: 0 },
+    })
+  })
+
+  it('shows a server validation error against the field', async () => {
+    signedIn({ 'POST /api/books': () => [400, { error: 'Unknown genre_id; see GET /api/genres', field: 'genre_id' }] })
+    const user = renderApp('/books/new')
+    await screen.findByRole('option', { name: 'Science Fiction' })
+    await user.type(screen.getByLabelText('Title'), 'Dune')
+    await user.type(screen.getByLabelText('Author'), 'Frank Herbert')
+    await user.selectOptions(screen.getByLabelText('Genre'), 'Science Fiction')
+    await user.type(screen.getByLabelText('Total pages'), '380')
+    await user.click(screen.getByRole('button', { name: 'Add book' }))
+
+    expect(await screen.findByRole('alert')).toHaveTextContent('Unknown genre_id')
+    expect(screen.getByLabelText('Genre')).toHaveAttribute('aria-invalid', 'true')
+  })
+
+  it('edits an existing book', async () => {
+    let saved = dune
+    const calls = signedIn({
+      'GET /api/books/7': () => [200, saved],
+      'PATCH /api/books/7': ({ body }) => {
+        saved = { ...saved, ...(body as object) }
+        return [200, saved]
+      },
+    })
+    const user = renderApp('/books/7/edit')
+    const title = await screen.findByDisplayValue('Dune')
+    await user.clear(title)
+    await user.type(title, 'Dune Messiah')
+    await user.click(screen.getByRole('button', { name: 'Save changes' }))
+
+    expect(await screen.findByRole('heading', { name: 'Dune Messiah', level: 1 })).toBeInTheDocument()
+    expect(calls).toContainEqual({
+      method: 'PATCH',
+      path: '/api/books/7',
+      body: { title: 'Dune Messiah', author: 'Frank Herbert', genre_id: 5, total_pages: 380, current_page: 142 },
+    })
+  })
+})
diff --git a/src/books/BookForm.tsx b/src/books/BookForm.tsx
new file mode 100644
index 0000000..421deb9
--- /dev/null
+++ b/src/books/BookForm.tsx
@@ -0,0 +1,153 @@
+import { useEffect, useState, type FormEvent } from 'react'
+import { Link, useNavigate, useParams } from 'react-router'
+import { api, ApiError, errorMessage, type Book } from '../api'
+import { card, inputClass, labelText, primaryButton, secondaryButton } from '../ui'
+import { useGenres } from './useGenres'
+
+type Fields = { title: string; author: string; genre_id: string; total_pages: string; current_page: string }
+type FormError = { message: string; field?: string }
+
+const EMPTY: Fields = { title: '', author: '', genre_id: '', total_pages: '', current_page: '0' }
+
+export function BookForm() {
+  const { id } = useParams()
+  const editing = id !== undefined
+  const navigate = useNavigate()
+  const { genres, error: genresError } = useGenres()
+  const [fields, setFields] = useState<Fields>(EMPTY)
+  const [loading, setLoading] = useState(editing)
+  const [loadError, setLoadError] = useState<string | null>(null)
+  const [error, setError] = useState<FormError | null>(null)
+  const [busy, setBusy] = useState(false)
+
+  useEffect(() => {
+    if (!editing) return
+    api<Book>('GET', `/books/${id}`).then(
+      (book) => {
+        setFields({
+          title: book.title,
+          author: book.author,
+          genre_id: String(book.genre.id),
+          total_pages: String(book.total_pages),
+          current_page: String(book.current_page),
+        })
+        setLoading(false)
+      },
+      (err) => {
+        setLoadError(errorMessage(err))
+        setLoading(false)
+      },
+    )
+  }, [editing, id])
+
+  function set(name: keyof Fields) {
+    return (event: { target: { value: string } }) => setFields((prev) => ({ ...prev, [name]: event.target.value }))
+  }
+
+  function invalid(name: keyof Fields) {
+    return error?.field === name ? { 'aria-invalid': true, 'aria-describedby': 'form-error' } : {}
+  }
+
+  async function submit(event: FormEvent) {
+    event.preventDefault()
+    setError(null)
+    setBusy(true)
+    const payload = {
+      title: fields.title,
+      author: fields.author,
+      genre_id: Number(fields.genre_id),
+      total_pages: Number(fields.total_pages),
+      current_page: Number(fields.current_page),
+    }
+    try {
+      const saved = editing
+        ? await api<Book>('PATCH', `/books/${id}`, payload)
+        : await api<Book>('POST', '/books', payload)
+      navigate(`/books/${saved.id}`)
+    } catch (err) {
+      setError({ message: errorMessage(err), field: err instanceof ApiError ? err.field : undefined })
+      setBusy(false)
+    }
+  }
+
+  if (loadError) {
+    return (
+      <div className={card}>
+        <p role="alert" className="text-red-700">
+          {loadError}
+        </p>
+        <Link to="/books" className="mt-2 inline-block text-indigo-700 underline">
+          ← Back to library
+        </Link>
+      </div>
+    )
+  }
+  if (loading) return <p className="text-slate-500">Loading…</p>
+
+  return (
+    <form onSubmit={submit} className={`space-y-4 ${card}`}>
+      <h1 className="text-2xl font-semibold">{editing ? 'Edit book' : 'Add a book'}</h1>
+      <label className="block">
+        <span className={labelText}>Title</span>
+        <input className={inputClass} value={fields.title} onChange={set('title')} maxLength={500} required {...invalid('title')} />
+      </label>
+      <label className="block">
+        <span className={labelText}>Author</span>
+        <input className={inputClass} value={fields.author} onChange={set('author')} maxLength={500} required {...invalid('author')} />
+      </label>
+      <label className="block">
+        <span className={labelText}>Genre</span>
+        <select className={inputClass} value={fields.genre_id} onChange={set('genre_id')} required {...invalid('genre_id')}>
+          <option value="">Choose a genre</option>
+          {genres.map((g) => (
+            <option key={g.id} value={String(g.id)}>
+              {g.name}
+            </option>
+          ))}
+        </select>
+      </label>
+      {genresError && <p className="text-sm text-red-700">{`Could not load genres: ${genresError}`}</p>}
+      <div className="flex gap-4">
+        <label className="block">
+          <span className={labelText}>Total pages</span>
+          <input
+            type="number"
+            className={inputClass}
+            min={1}
+            max={100000}
+            value={fields.total_pages}
+            onChange={set('total_pages')}
+            required
+            {...invalid('total_pages')}
+          />
+        </label>
+        <label className="block">
+          <span className={labelText}>Current page</span>
+          <input
+            type="number"
+            className={inputClass}
+            min={0}
+            max={fields.total_pages || undefined}
+            value={fields.current_page}
+            onChange={set('current_page')}
+            required
+            {...invalid('current_page')}
+          />
+        </label>
+      </div>
+      {error && (
+        <p id="form-error" role="alert" className="text-sm text-red-700">
+          {error.message}
+        </p>
+      )}
+      <div className="flex gap-2">
+        <button type="submit" disabled={busy} className={primaryButton}>
+          {editing ? 'Save changes' : 'Add book'}
+        </button>
+        <Link to={editing ? `/books/${id}` : '/books'} className={secondaryButton}>
+          Cancel
+        </Link>
+      </div>
+    </form>
+  )
+}