It depends on whether you feel you have a consistent approach to the basic problems that you can call up any time and you know how to identify the concepts invovled. For instance when you read a math problem you should be able to identify the word which triggers a concept and then remember a basic and consistent approach to tackling that type of problem. If you cannot do that the it may help to continue studying the books. However, you should remember to treat teh information in a hierarchy - not all information is equally important.

If you have a consistent approach to the concepts tested on the GMAT then you should spend more time reviewing. in each question look for that trigger words, identify the concept and review it - then keep a tally of what in the problem tricked you - write it down. After you have done all of the above re-do the problem adn make sure you can identify all of the above in it.

Hope that helps - Happy Studying.