Any GMAT prep book from a reputable company will cover the essentials. GMAT math is fairly basic, in that the math itself is usually straightfoward - it's how the questions are worded that makes it tough.

When I say "basic", I do mean it - the vast majority of problems only require simple arithmetic, algebra, and geometry. There is no calculus, or even precalculus or trig. So, in addition to a GMAT book, some people like to get general math books (i.e., not specifically written to the GMAT) that cover those high school math topics.

Most of your effort is likely going to be in learning how to decipher questions, not the actual math itself.