Enter the ISBN number into the ISBN search engine on any book search website. There are several of these, including ones associated with particular booksellers like Borders or Barnes & Noble, which will then allow you to purchase the book online.
If you have at least a partial copy of the book, locate the ISBN beneath the barcode on the back cover, or on the same page as the copyright information. Finding the book by ISBN can then supply information that may be missing, such as the title, the author or the publisher.
Go to a local library or bookseller and ask them to look it up for you if you're having trouble finding it online yourself. Libraries and booksellers have easy access to ISBN databases that will help them get you the information you need fast.