From what u have said i assume that u need to build a solid foundation in quants. Here's what i would suggest :
1. Kaplan GMAT Math Workbook - A solid supplement to the Official Guide
Strengths:
a. Fairly comprehensive review of the basic math concepts needed for the GMAT
b. Structure of the book allows you to work from start to finish, building on previous skills learned
c. Sections devoted to Word problem and Data Sufficiency questions
Weaknesses:
a. Statistics, Combinations/Permutations and Probability are not covered
b. Questions do not reflect GMAT questions, but rather are designed to insure that the material is mastered
2. Manhattan GMAT Foundations of GMAT Math
Strengths:
a. Best book for brushing up on long forgotten math concepts (fractions, powers, etc)
b. Great for refreshing math concepts and building confidence in math
Weaknesses:
a. The book has 400 practice questions but they are not in a GMAT format, rather a math-textbook format
3. Math Review For Standardized Tests (Cliffs Test Prep)
Strengths:
a. Great for a zero-knowledge start or for those who did not deal with maths problems for a long time.
b. Simple,straightforward and relevant
c. Extremely low price
Weaknesses:
a. Technically not a GMAT-dedicated book
b. Lacks some of the more advanced areas
4. Manhattan GMAT Math Guides - All 5
Strengths:
a. These books don't emphasize tricks or easy ways to 700+ score, instead they methodically cover areas tested, and should be pretty much all you will need (besides the Official Guides).
Weaknesses:
a. As with all MGMAT Guides, lacks practice questions and relies on the Official Guide for additional practice.
Hope this helps : )
