The International Standard Book Number (ISBN) is a unique 10 or 13 digit numeric commercial book identifier publishers purchase to identify separate editions and variations of a books, eBooks, tapes, and CD’s. The ISBN numbering system is used worldwide by publishers, booksellers and libraries for ordering, listing and stock control activities. Many online publishers offer ISBNs for free but you may encounter issues later if you want to reprint, republish or sell your work (movie, audio, etc.) as the ISBN is owned by the publisher. They are expensive, but as the author you want to always retain your rights.