GRE materials

In my opinion, every student needs The ETS Official Guides to the GRE, which contain scores of original questions, two complete paper-based tests, and two complete computer-based tests. Since the language of GRE questions is quite particular, it's important to get accustomed to it. Third party companies (Manhattan Prep, Kaplan, Magoosh, etc.) can only imitate questions with varying degrees of verisimilitude, and there is a difference. 

This actually weighs 5.2 pounds, so toting it around town will tone your triceps, too. This book is one of the best deals around, containing 1000+ GRE-type quantitative reasoning (math) questions organized by topic with thorough explanations.

While the 5LB and official guides provide answer explanations for spot review,  they are not written to teach topics, such as statistics or divisibility, from scratch. If this is required, 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 verbal question types.