Required books for GMAX Online courses:

GMAX suggests that you study from:

The 12th Edition of The Official Guide for GMAT Review, (ISBN
9780-470-7445-12)

The Official Guide for GMAT Quantitative Review, 2ND Edition, (ISBN:
978-0470-7474-45) 

The Official Guide for GMAT Verbal Review, 2nd Edition, (ISBN:
978-0470-7474-52)

Note: The Official Review Books mentioned above are taken directly from the 10th Edition of the Official Guide, but do not cover all of the questions.
There are many questions from the 10th Edition that have been left out of the newer books. Many of these questions are more difficult than those in the new books, which is why we suggest you use both editions of the Official Guide.