GMAT materials

In my opinion, every student needs the The GMAT Official Guide bundle, which contains hundreds of original quantitative and verbal questions. Since the language of the GMAT is quite particular, it's important to get accustomed to it. Third party companies (Manhattan Prep, Kaplan, Veritas, Magoosh, etc.) can only imitate questions with varying degrees of accuracy, and there is a difference. Toting these tomes around town will tone your triceps, too.

Foundations of Math is a very important book for many students because it provides straightforward drills on important mathematical operations, such as percents, exponents, roots, and geometry.    In my experience, slowness and inaccuracy in the execution of basic mathematical operations can be as much a problem for some students as higher level concepts can.

What the Official Guides DON'T provide, however, are very useful reviews of the quantitative and verbal concepts on the GMAT. I will usually recommend  The Manhattan Prep set of books, which provide a thorough and well-structured review of all quantitative topics and of each of the three verbal question types.