What is the difficulty level of a PMP examination?

The PMBOK is enough in my humble opinion. Lot of books contains interpretations of the PMBOK which is not good and do not help. All people in this world are performing project management from the time they wake up to the time they go to bed. So, think about anything you do in your day and tried to map the PMBOK to it. If you still search for a book take a look to "Visualizing Project Management",

Savrabh, the PMP exam has a well-earned reputation for being difficult. Many of the questions are situational. And even though the questions are multiple choice, there are often two answers that could be correct. These are the most difficult questions, in my opinion, because you have to choose the "most correct" answer.

I was fortunate to pass on the first attempt. If I had to take the PMP exam again, I'm not sure I'd pass every time. I've known some very good project managers with years of experience who failed on their first (and even second) attempt.

1st of all you have to understand the concept of each process (49 processes in PMBOK 6), It not help to remember the definition of each process instead you have to make sure how you can use each process in real projects in PMI way, practical experience may differ some time but you have to think as per PMI standards and ways to pass the exam. Need a lot of study on practice on mock questions, Exam Simulations. 95% of the questions are situational and two answers are very close, so make a group of two to three members for discussing the understanding of the concept. Also, most of the question will be asked indirect way so practice well to get succeed in the exam.

Experienced people is the people that can fail the exam more easily. Why? Because usually they answer the questions based of their experience instead of asking the questions based on what PMI expect as an answer.

In my opinion, experience is not enough especially when your organisation dont use PMI standard.
I think the key is to understand the philosophy of each of the 49 processes and how they interrelate. On the exam, you need to find out which process is the question about, and only then you can answer. The most difficult questions are those that have two or more answers and you'll have to select the best one according to the standard philosophy. For that, training questions can be helpful.
Another point, you must be careful on choosing the training, some of them are outdated and their questions are so easy that they will only help you get the basics of project management, but in fact they have nothing to do with the actual exam difficulty.