What is an ISBN number, and do I need one?
You will need an ISBN number and bar code if you want to sell your book in any kind of bookstore, either bricks and mortar or on the internet.
The ISBN number ties into a catalog of all (registered) books in print. This catalog lists the publisher, access and purchasing information, and descriptive data about each book.
As a grammatical comment, “ISBN number” is redundant; “ISBN” means “International Standard Book Number”, so “ISBN number” is “International Standard Book Number number”. But it’s common to say number anyway.
If you are selling your book on your own, you are not required to have an ISBN. If you want to sell your book in bookstores, place it with distributors and wholesalers, and put it in libraries, you are required to have an ISBN on your book.
(source: signature book printing)