How to continue after the first preparation book?

I think you should take an actual computerized practice test to see where your weaknesses are; it sounds like you took a paper-based test. Obviously, getting 90% right is excellent - but it would be good to see how that translates into a CAT score.

I see that you're German. My experience traveling in Germany tells me that your English is probably better than that of many native English speakers! In any case, if you're worried about it, I'd recommend just getting practice reading and writing in English as much as you can. Start reading novels, magazines, newspapers, or whatever else you can in English. That's the best way to set a solid foundation for verbal.

As for practice problems, the OG is defintely the best source of materials. There are two other OGs as well that you should buy if you haven't already. They are green (quant) and purple (verbal). Those problems are very accurate unless you're scoring over 90th percentile in math. If that describes you, you'll need to get some other materials. Also, note that the books go in approximate order of difficulty, so don't get too confident if you get all of the first 1/2 of each section right - the true test is in the latter problems.

Here's a link to free Veritas practice test. After you take it, let us know how it went and where your weaknesses were.

https://www.veritasprep.com/gmat-prepara ... tice-GMAT/

Good luck!