The GMAT Review is the official guide by GMAC, the company that administers the test. (Most abbreviate it as OG.) Although you may use other books as supplements, you will want to make this official guide the core of your GMAT studies. GMAC also has two additional texts for verbal and quantitative that you may also want to consider.

If you get these books and find out that the problems are too advanced for your current knowledge, you may need to go back and refresh your knowledge on fundamentals using other sources, depending on your areas of weakness. The OG doesn't go too far in depth in explaining basic math or grammar, but you will absolutely need to learn how to do the problems in that book, and learn how to apply what you learn to similar problems.

the OG is a must, as mentioned before. I also have the new kaplan book and it's pretty good. I got that and the manhattan review books off of amazon.