GMAT For Dummies
Scott Hatch, Lisa Zimmer Hatch
For this price, this is a great buy and you can't go wrong with it. Of course, this is not a book that you can completely rely on, but you may end up learning a few new things. You'll definitely need additional help. So, use this book purely as a supplement. For additional help, consider getting the Kaplan or Princeton Review books. If you want more focused and detailed study guides, consider EZ GMAT series and/or Manhattan.