There are probably hundreds of GMAT books on the market. Your starting point should be to learn what GMAT says about its own test. In other words, start with the GMAT books and software published by GMAT. This does NOT mean that you should use only materials from GMAT. It means that you should start with them.
These books and GMATprep software are on the GMAT site at:
The are also included with all courses offered by GMAT Preparation Courses Canada.