Hi afdebbas,
First off, a 590 is above-average, so considering that you appeared to have studied from just one book, you performed about as well as can be expected. Before you purchase any more books, it's important to define a few things:
1) How long have you studied?
2) What is your goal score?
3) When are you planning to apply to Business School?
4) Have you thought about which Schools you want to apply to?
5) How much time have you given yourself to continue studying?
Thankfully, the GMAT is a predictable, standardized Test, so you CAN train to score at a high level.
GMAT assassins aren't born, they're made,
Rich