GMAT Official Guide is a comprehensive guide for the entire GMAT Exam consisting of all the sections - Quant, Verbal, AWA and IR. It introduces the GMAT Exam and also shares some test-taking strategies. There are two books of GMAT Review - Quantitative Review and Verbal Review which are provided so that you get to practice more number of questions in these sections specifically.
The theory and the concepts provided in the Review Books are the same as the those provided in the Official Guide.
There are about 300 practice questions in each of the Quantitative and Verbal Review Books and about 900 practice questions in the Official Guide.
Solving these books is a must for the GMAT Exam.