The number one book we recommend is The Official Guide. It's the only textbook written by the same people, who make the actual test, so it's a great resource. They also published two additional books that focus specifically on Verbal and Math Sections. If you only want to buy a few books, I would start with those. You can buy all of them on amazon:
For additional resources, we recommend the Manhattan series. It's a set of 8 books, each focusing on a different subject (for example, a complete book is dedicated to algebra. Another covers quantitative comparison and data interpretation). Because the series is broken down by topic, you can pick and choose which books you want to buy, depending on your strengths and weaknesses. You'll also receive a code with the book that allows you to access a free practice tests online. You can read our review of the book on our blog. The review is a comprehensive look at the Manhattan series of books. It was written in 2012, but does a great job of laying out the books:
For the most up-to-date review of the Manhattan books, you can read these articles:
For some tricky math questions check out the Nova's GRE Math Prep Course. It is an excellent source for more challenging quant questions, which will thoroughly prepare you for the harder questions you see on the exam. It actually tends to be even harder than what you usually see on the test! And finally, the GMAT Official Guide is a pretty good source for hard math problems as well. The format will be a bit different, but the difficulty level is similar.
You can read the rest of our book reviews here:
All of those books are not required to score highly on the test - Magoosh by itself is enough for you to do well. However, if you want more practice, those would be great!