Hi Tomas,
To start, 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 non-book resources. By extension, you might not want to invest too heavily in a lot of books at this point.
Since it sounds like you're just beginning your studies, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can take 2 for free at
www.mba.com (and they come with some additional practice materials). If you want to do a little studying first, so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can come up with a study plan.
I'd like to know a bit more about your timeline and goals:
1) What is your goal score?
2) Are you considering any other Schools besides Harvard and Stanford?
GMAT assassins aren't born, they're made,
Rich