Hi a22mun,
Based on your initial post, it's not clear whether you're just beginning your studies or you have put in some significant study time already. Many GMATers use a variety of different study materials - some books and some online (courses, practice CATs, etc) - so whether you buy these books or not, you will almost certain end up investing in some addition non-book materials.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich