This is a commonly asked question. Quite simply, ISBN in an abbreviation for “International Standard Book Number.” An ISBN is a unique 13 digit numeric identifier for a book. By utilizing an ISBN, all booksellers can access information about any book by simply referencing the book’s ISBN. Think of it like a serial number or a social security number for a book. An ISBN commonly references the author of a book as well as the book’s publisher. In the United States, only one company- RR Bowker- has the ability to issue an ISBN. ISBN;s typically costs $125 each, and it is not uncommon to need two or three ISBN’s for one books, since each “edition” requires its own ISBN. Thus, if you are releasing a book in hardcover, softcover and ePub format, you will need three separate ISBN’s. Read more technical details here.