ISBNs are 13-digit numbers used by publishers, retailers, and libraries to identify and track books. No two books have the same ISBN—every format of every book (eBook, paperback, hardcover) has its own unique ISBN.
Gatekeeper offers a free ISBN with each of the following services: eBook Conversion & Distribution, Paperback Layout & Distribution, and Hardcover Layout & Distribution.
If you prefer to have use your own ISBN, you can purchase an ISBN for $125 through Bowker, the U.S. ISBN agency. (If you are located outside the U.S., ISBN prices vary depending on your local agency.)