1. Cracking the GMAT
by Prinction Review
Read this to gave a sense of what GMAT test entails, and some basic but useful strategies.
2. Official Guide for GMAT
This contains past questions used in past GMAT test. Use it for practicing various sections.
These 2 were what I used more intensively for my prep. The other book that I used what Kaplan
's GMAT. However, I use it more for the full scale prep tests that it provides.
Remember to download Powerprep
/GMATprep from mba.com - these are pretty essential as part of your prep too.
All the best.