I'm extremely familiar with the Head First PMP, the PMBOK, and Rita's book.
If you are looking to prepare for the PMP Certification
test, or just understand project maangement best practices in general, the Head First book really helps you get your mind around the whole process and have it make sense. It also uses the most advanced adult learning theory to help you retain the information without a lot of rote memorization.
Then, you will need to move to the PMBOK (A Guide to the Project Manager's Body of Knowledge) for the nitty, gritty of details that could be on the test. But without the Head First PMP knowledge, the PBMOK can be very confusing.
Rita's book is a more traditional approach to project management.
B. Davis