I attended an AICP prep course in 2006, and the most useful thing that came from it was the admonition to always remember that the CPE is not a planning exam; it is a membership exam. They don't care about one's expertise with CEQA or the SMA. They're not impressed with how much a person might know about planning nearly as much as they are with how much that person knows about what APA and AICP want to hear.
Despite the huge number of fantastical planning tomes that are recommended reading for prospective AICPers, I found (and have tried emphatically to impress upon others) that practice exams are the most helpful tool in preparing for the CPE. As far as specific topics, practice exams can tell a person what fields could be covered on the test; frequent reference to a particular item over the course of several exams should be a decent indicator that it's touching on a subject important to the test-writers. More importantly, practice exams help a person understand that on the CPE, the way the question is asked is often as important than what it's asking. These exams offer the opportunity to not only find out what the right answer is, but why it's the right answer and why the other choices weren't quite as right.
While a person could go crazy even skimming the 1,643 texts suggested by AICP leading up to the exam, practices tests can help narrow the focus. I'm not suggesting this as sort a Cliff's Notes version of CPE studying. It's more of a "Well, you could read and memorize all of the stuff in all of these books, or you could look at the kinds of things that actually show up on the exam and tailor your studies that direction, instead." Also, at least the way that I operate, looking up one thing following a failed exam question leads to looking up something else, and so forth. This can result in a great amount of useful research that feels spontaneous and self-directed rather than forced (i.e. poring over dusty grimoires by candlelight).