You should be running the final version of the Book Reviews plugin created in Chapter 4, The Power of Custom Post Types, and should have already followed the Creating a client-side content submission form recipe. Alternatively, you can get the files from the code bundle (Chapter 4/ch4-book-reviews/ch4-book-reviews-v11.php and Chapter 6/ch6-book-review-user-submission/ch6-book-review-user-submission-v1.php), and rename ch6-book-review-user-submission-v1.php to ch6-book-review-user-submission.php.