Take a practice test before you select a specific test prep book. A practice test can help you diagnose your weaknesses and decide what areas you need to study.
Choose a book that addresses your specific weaknesses. For instance, if you scored poorly on the math section, you may want to select a book that focuses on reviewing math material. Some books also offer help with general test-taking strategies, like improving your use of time or smart guessing if you can't find the right answer.
Pick a book with extra practice tests, or purchase a practice-test book. Taking additional practice tests can improve your score further, as well as provide you with a way to track your progress. Keep in mind that some books only have sample questions, not full practice tests.
Avoid books that use confusing techniques or designs. Instead, select a book that is easy to understand and use. You should be able to write in the book, practice problems and take notes as well.
Follow your book's structure to make the most use of it. Most books are written to proceed through progressively harder material, so they can be harder to use if you skip around.