As for study materials.
Everythying Official Guide goes without saying. The Orange book (comprehensive), the purple book (verbal) and the green book (quant)
Verbal:
1)
Manhattan GMAT Sentence Correction Guide (
MGMAT SC Guide)
2) Powerscore CR Bible (GMAT Version, LSAT version if you need even more)
Quant:
1)
Manhattan GMAT math books. Manhattan publishes books such as Number Properties, and others. I can't remember more offthe top of my head. Also, when you buy one of their books for $26, you get 6 free CATs and they're great CATs.
2) Paul Dawkins Downloads. These are essentially eBooks written by Paul Dawkins, math professor at Lamar University in Teaxs. They are amazing (in my opinion). I like diffential equations and Algebra I.
3) Any post by username: walker. He's a math genius. There are others on here, but he stands out in my mind. He's also very, very friendly and helpful. If you have a question, send him a PM and he'll write you back.
4) Last, but certainly not least, the GMATClub Challenges. You can get them for free. Check out the pricing page to see what the criteria is that you need to hit in order to get them for free. It's about contribution to the club that you may or may not have time to do.