How to continue after the first preparation book?

Hey guys,

I just finished 'Cracking the GMAT' by Princeton Review which was actually really helpful to get started. Made a summarization (approximately 40 pages) about everything.

After that I tried the diagnostic test (only quant. section so far) by GMAC and answered 90% of the questions correctly. Is this test a good indicator for my actual score on the GMAT?

And which book should I start with next? (have learned 1 month so far ; 3 more are left) Thought about Kaplan or continuing with the OG. Verbal books should be considered as well since English is not my native language, but I think getting used to the basics first is the best way. Are there any books that only contain difficult (600+) questions?

Thanks for your help
Sandro

take as many practice tests as possible ... to get required stamina.