The PMP Exam is mainly based on A Guide to the Project Management Body of Knowledge (PMBOK Guide). The most current version was published in 2008 and most people find a bit difficult to read. But due to the fact that the exam is based on this book, it is important that you actually read and understand all that it contains. As a supplement to studying the PMBOK Guide in preparing for the exam you need to get additional reference material.
I recommend that you buy a separate PMP Exam Prep book that will further enhance your understanding of the subject matter. These books will help make the complicated concepts from the PMBOK Guide clearer and they also give you more information and tips on how to prepare for and pass the PMP exam.
So here is my tip: read and understand the concepts from the PMBOK Guide and from a separate PMP Exam Prep book. Also work through the sample questions and exercises in your prep book. And lastly, take as many sample exams as you can to gauge your understanding of the concepts. Go back and review the sections in the books that you did not score well and try again.
When you start scoring above 80% the first time you take any mock exam then you are ready for the real exam. Remember that it takes a lot of preparation to make sure that you pass your PMP exam and receive your certification. So study hard, learn well and be confident of your ability to get your certification.