Hi veeral,
There are a variety of ways to prepare for the GMAT, so until you define your goals, strengths and weaknesses, it's tough to say for sure what YOU will need to work on to properly prepare for the GMAT.
Most Test Takers use the Official Guides in some capacity (as those books include actual questions that once appeared on the Official GMAT), so those are a good investment. Any additional suggestions will be based on how YOU learn best, your timeline, practice scores, etc.
1) What is your goal score?
2) What studying have you done so far?
3) Have you taken any practice CATs? If so, then what were your scores?
4) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich