Hi nphsingh,
Those books are great supplemental study resources (since they each include a few hundred retired GMAT questions), but they are necessarily a required part of your studies.
1) What is your score goal?
2) How have you been scoring on your practice CATs (including the Quant and Verbal Scaled Scores)?
3) When are you planning to take the GMAT?
When it comes to studying for the GMAT, there are a variety of different resources that you can work with. Most GMAT Companies offer some type of free materials (practice problems, Trial Accounts, videos, etc.) that you can use to 'test out' a product before you buy it. We have a variety of those resources at out site (
www.empowergmat.com). I suggest that you take advantage of all of them then choose the one(s) that best matches your personality, timeline and budget.
GMAT assassins aren't born, they're made,
Rich