There's a Lot of Material - Here's Your Roadmap to Studying for Your PMP Exam
The Project Management Body of Knowledge is the primary source for the questions on the PMP exam. It isn't the sole source, but it's the major source. Exam prep courses, textbooks and other tools are based primarily on the PMBOK. Through March 25, 2018, the PMBOK Fifth Edition is the source for many of the questions on the PMP exam. A new PMP exam arrives on March 26, 2018. Its primary source is the PMBOK Sixth Edition, That edition is in print now.
Several people have written PMP exam prep textbooks. My personal favorite is "The PMP Exam: How to Pass It on Your First Try," by Andy Crowe. It was one of the tools that helped me pass my PMP exam in 2011.
PMI and Independent Publications
The Project Management Institute publishes several white papers, reports and books. Some are free to the general public. Others are available only to PMI members.
Many independent authors have published articles containing tips to pass the PMP exam. Your favorite search engine can deliver hundreds of suggestions.
Your Greatest PMP Exam Prep Tool: Practice Exams
Take many practice exams. When you can score 80%, three times in a row, you are probably ready to sit for your PMP exam.
A past president of PMI-Portland took ten practice exams before he sat for his real PMP exam. Of course, he passed.