The PMBOK Guide is a must-read book for the PMP certification exam. There is no doubt that you must read the PMBOK Guide several times before going to 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.
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 decide which book to purchase.