For learning/refreshing the math relevant for GMAT:
- Manhattan GMAT guides - detailed explanations but too few practice problems
Veritas: Combinatorics and Probability - the best book in that particual topic
For practice problems:
- Official Guide
Quantitative Review
Princetone Review 1037 Practice Questions - the closest in style to the official problems
Princeton Review 1014 GRE Practice Questions - GRE and GMAT essentially target the same math
Veritas books
Nova books
VStudy books
For practicing under timed conditions:
- 800 Score CATs
MGMAT CATs
the official GMATPrep
the official GMAT Focus