What usually happens is that students preparing for the PMP exam try to go through too many PMP Exam Prep books or on-line courses thinking that they will be able to absorb everything that they watch and read. That is far from the truth. Too many books will in fact confuse you. Usually, one good exam prep book and one course is enough to complement the PMBOK Guide. Reading the PMBOK Guide is of course a must for any PMP student and no exam prep book is a substitute for it. And don't forget to read the the PMBOK Guide Glossary.