ISBNs (International Standard Book Numbers) are unique product identifiers used by bookstores, publishers, libraries, and others in the business of selling books. The rule of thumb is: If you plan to sell it in some format, you will probably find it useful to have an ISBN. If you don’t plan to sell your book, you don’t need an ISBN. Many people publish open access books with Pressbooks, and they typically don’t need ISBNs, unless they plan to sell the book in some formats (such as a hard-copy printed book). For authors who choose to apply for an ISBN, they can enter this number in the book metadata for their Pressbooks-published work.