OG = Official Guide are the guide(s) that GMAC puts out.
Books Available:
Official Guide 11
- Newest book, tons of math and verbal questions
Official Guide 10
- Older book, tons of math and verbal questions
Official Guide Verbal Review
- 300+ more verbal questions
Official Guide Math Review
- 300+ more math questions
In my opinion the verbal sections in all of the books are great. The math stuff is valuable too, but the first half of the questions in each book are not realistic as far as difficulty goes. In all I would highly recommend getting some combination of the guides.
Hope this helps!