ISBN stands for International Standard Book Number. It is a 13-digit number that uniquely identifies media products (such as books or audio cds) published word wide. ISBN’s are important to identify your book so it can be marketed at a variety of retailers, such as libraries, universities, wholesalers and distributors. There are over 160 agencies worldwide where you can purchase your ISBN, however each geographic territory will only have one agency where you can purchase your ISBN. Once ISBNs have been assigned to products they should be reported to R.R. Bowker as the database of record for the ISBN Agency in the US.
ISBNs are sold either as 1 or in blocks of 10’s. I recommend you get at least the 10 block, but really look at your situation. It is best to purchase what you think you’ll need for the next couple of years. It is also important to remember that you may need more than one ISBN for a single publication. That is because each format you distribute needs its own ISBN. For example, hard copy, soft copy and e-reader versions would all get their own ISBN number for the same book title.
If you’ve already published your book, but didn’t purchase an ISBN number, that is ok. You can purchase one and apply it on your next print by affixing the ISBN barcode to the back cover of the book.