Generally, it is the handout materials provided by PMBOK and training center. If you have enough time and are willing to invest more, you can buy some other books yourself, such as Dr. Wang's interpretation of PMP exam, headfirst and so on. Or you can borrow some related counseling books from the nearby library.

However, remember that PMBOK (20 17) is the fifth edition. When buying and borrowing books, don't take the previous edition.