If you plan to sell your book in bookstores, to libraries, or through online retailers you will need an ISBN. There are several reasons why you might not want to use a free ISBN number for your book, including:

The ISBN (International Standard Book Number) is your book’s identification number or fingerprint. It is a 13-digit number, not a bar code. ISBNs are calculated using a specific mathematical formula and include a check digit to validate the number. ISBN information is embedded in the bar codes on the backs of books.