If you want one online, go to 4shared.com and make an account if one is required. Select 'Books' and type GRE in the search bar. You'll find plenty of books, Princeton Review, Kaplan, Nova, Barrons etc. All books are good.

If you can grab one from a bookstore, Dummies guide to GRE is an excellent excellent resource that simply stands out. If you find it online, don't forget to share it with everyone!