Stephanie,
This is a well kept PMI secret and nobody really knows. Let's look at how the questions are created:
PMP Exam questions are created by a worldwide body of volunteers. These volunteers create questions that are based on their own experience as well as books and other reference materials that they have read. These questions are then reviewed by a PMI team, approved and then added as non-counting "pre-test" questions into the exam. In this way, every exam taker will become a guinea pig and answer 25 of these pre-test questions. When a question does well in the pre-test, then it is incorporated into the real PMP exam.
This much to how questions are generally speaking developed and integrated into the test.
Now that you know the process you can see that it is pretty much impossible for anyone to really know what books the questions are coming from. Yes, you will be able to find "recommended reading lists" out there but I wouldn't trust them too much. We always recommend that PrepCast students read the PMBOK Guide, work their way through 1 PMP Prep Book and take as many sample questions as possible.
