Hi imnik,
To start, you've given yourself plenty of time to focus on the GMAT before you apply to Business School - which is good.
Many Test Takers are unhappy with their initial practice CAT Scores, but you really shouldn't be - that's just a measure of your skills at that point in time and you'll likely improve as you learn more about the Exam and the Tactics that you can use to pick up points and save time. That having been said, raising a 500 to the point that you can consistently score 700+ will likely require at least another 3 months of consistent, guided study - and you'll have to make significant improvements to how you handle BOTH the Quant and Verbal sections. Thankfully, the GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level.
It's also worth noting that many Test Takers who use a 'book heavy' study approach end up getting 'stuck' at a particular score level, so you'll likely end up needing to invest in some new non-book resources at some point. You can certainly work through the books that you currently have though - and I suggest that you do so. In 2-3 weeks, you should take a NEW, FULL-LENGTH CAT and we can use that result to assess how well you're improving and discuss how you might adapt your studies going forward.
GMAT assassins aren't born, they're made,
Rich