Kaplan GRE Exam Math Workbook 7th Edition
This book offers a good overview of the math tested on the GRE General Test. It's essentially an overview of math concepts learned in high school - but this is the level of math tested on the general test. (Only the GRE Math Subject Test tests more advanced concepts like calculus.)
If you are like me and have not taken any sort of math class in years and would like a refresher in Algebra and Geometry to prepare for the GRE General Test, then this book is a good place to start.