The International Standard Book Number (ISBN) is a unique numeric commercial book identifier. We offer both the number itself and generation of your barcode in various formats.
An ISBN is recommended but not required for any ebook you publish through most online stores, although it is required for print editions.
Note, you can't use an ISBN across different formats, it is assigned to each edition and variation (except reprintings) of a book. For example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN.