GMAT - Graduate Management Admission Test


I'm taking the GMAT 3 months for now at least.
English is not my native language, so I know that I need to improve it a lot. I find myself at ease with math (I'm studying economics, so I've got to able to interpret graphs and do mental calculations fairly quick), though a revision in some areas, such as geometry, is very much advisable.

My doubt is: apart from the official guide from GMAC, what books should I buy? Manhattan? Kaplan?

By the way, I'm gonna take the IELTS test too, so I believe I can prepare myself for both tests at the same time, since the improvement in my english induced by the preparation for the IELTS will help me in the verbal component of the GMAT.

Thanks for the input guys.