navigator, ISBN stands for International Standard Book Number. It's a unique number that is assigned to every book published. Different editions of a book will have different ISBNs. So the paperback and the hardback of a book would be different.
I know that you can find ISBNs for books that are listed on
www.amazon.com . I'm sure there are ways to find books that amazon doesn't list, but I'm not familiar with it.
Let us know if you find what you need. Someone else will probably come along with more help if you need it.