There is no doubt that you must read the PMBOK Guide several times before going to the PMP certification exam. The PMBOK Guide is a must-read book for the PMP certification exam.
That being said, the PMBOK guide is the basis for the PMP certification exam; however, to pass the exam you also need to refer to some other PMP exam preparation books because the PMBOK Guide does not answer all questions asked in the test; e.g. situation-based questions.
Moreover; to understand the project management concepts and various tools and techniques, you have to study some good reference books and practice the exam questions.
I have listed some famous books in the PMP reference books section of this blog. You can review those books, discuss with your friends, and then make a decision of which book to purchase.