As with any test prep, it's best to get multiple books with practice tests because you need them to gauge your strengths and weaknesses-it's best to splurge for the latest versions of tests. If you're taking yours online, it's best to get online practice tests and see how you react to the pressure because it's different from doing it on paper.
The GMAT is particularly hard on math because of the business focus on math in particular so make sure your math foundations are strong. Depending on your score, you can start focusing on what program and where you want to apply for your MBA. Some people only need a score to apply. Others want to go to the best schools but remember, you get what you pay for. In the end, some hiring managers only require an MBA degree and don't need you to go to the best school to get it.