Hi Dominik,
Your question assumes that you should be using books as the primary source of your GMAT training - and that might be a faulty assumption.
The GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level. The immediate question is whether a book (or books) will offer you the proper training to hit your Score Goal. To raise your scores, you will likely have to do more than just work through a lot of practice questions. Most GMAT questions can be answered in more than one way, but if you're not properly learning the content, patterns and Tactics for the GMAT, then there will likely be a limit to how much you could improve.
Thankfully, you have lots of time before the May 1st application deadline you mentioned, so you can proceed with your studies however you like. If you want to try focusing on the Official Guide for now, then you can - and you can track your improvement by taking FULL-LENGTH CAT Tests under realistic conditions. If you end up getting 'stuck' after a month or two of working in this way, then you can change your Study Plan and still have plenty of time to improve.
GMAT assassins aren't born, they're made,
Rich