The ISBN is a number issued to each book by the publisher and embedded in the bar code on the back of the book. The number comes from R. R. Bowker, the official U.S. agency responsible for assigning ISBNs. The system they designed makes it easier for everyone to order books.
In order to sell through bookstores, distributors, wholesalers, and major retailers, each book must have a unique 13-digit ISBN.
Every publisher must assign an ISBN for each new book and each new edition of the book including ebooks.
ISBN numbers can be ordered singlely, or in blocks of 10 or more for about $270 for the first 10. The cost per book goes way down with the more numbers ordered.