I haven't been using all that many books for the GMAT. Unlike for the LSAT where I went out and purchased prep books and materials, I'm only using the ones that are at my library for the GMAT.
As in a previous post, take the LSAT first, b/c then most of the verbal section of the GMAT will be a breeze. Then if you need to polish up on your math, I would recommend the GRE/GMAT Math workbook from Kaplan. Its goes over most of the math you need for the exam and it has tons of practice questions. I liked this book b/c it had been a couple of years for me since I had done some serious math. This book helped me to catch up. As an overall starting point for the GMAT check out Cracking the GMAT by Princeton Review. This book won't make your score go up by tons of points but its a good starting point to find out where your weaknesses are and what you should work on. To get actual real GMAT questions, you can buy the Official Guide for GMAT Review from GMAC or at any bookstore. I don't have that book b/c honestly, I'm not as worried about this exam as I was about the LSAT. If you do a Google on GMAT practice exams, you'll find tons of web sites with free practice questions. That is the extent of my studying for the GMAT.