ISBN stands for International Standard Book Number. It is a 13-digit number that’s used internationally as a unique identifier for books. ISBN also identifies the publisher of the book. It is the standard ID number used to identify books by booksellers, libraries, book wholesalers, and distributors. If you plan to sell your book in bookstores, to libraries, or through online retailers like Amazon.com, you will need an ISBN. ISBNs are issued to publishers, who then assign them to individual books. This can be done at any time, even before the book is written. ISBN is mandatory if you wish to list and sell it via retailers; if you wish to have the book only for private circulation the ISBN is not necessary.