This is a good place to start if you really need to work on the basics. This book is can be a refresher for some people and it can be used as way to learn some of the basic math if you don't have a math background. it gives some good explanations.

However i think it should be used only as a supplement to other gmat books (i.e Kaplan, Mgmat etc.), since i'm not sure how closely the practice questions resemble actual gmat math questions. Again this book is mostly for content and not much for test strategy.