It takes months to improve 100 points if you are 550-600 range, You just cannot score high without doing official questions, you will realise this when you prepare well. Check the books what 700+ scoring people have been reading from this website Ibeatthegmat.com forum & GMAT Experience from GMATClub , you will realise one set of books are not enough . Giving GMAT is not like TOEFL if you are non-native then you are weak in verbal, if native weak in quant. I had to work hard for both though, make a strategy after you take Official Mock test, dont waste an official attempt even if you are in hurry. I have been in same boat scored 500 just after college in 2007 without much prep and again scored 520 in my last attempt in November where I prepared mostly for quant, this would be my 4 attempt now I have been working on verbal , buy ExamPack 1 from mba.com for 2 more official Tests. Best of Luck

Throw in another agreement with Rich and Brent. Take a practice test as soon as you can, and spread them out during your studying (don't do them all at the end). They're great for working on pacing, strategy, and stamina, but they're not great for learning the content.

Thanks very much for all the information.