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.
First of all, thanks for posting such nice things about "Head First PMP". We're really proud of it, so it's really great to see that others appreciate it too.
Our technical review team for "Head First PMP" actually included two of the people who wrote the PMBOK(r) Guide, including the person who was the project manager for creating the PMBOK(r) Guide 3rd. Edition. I actually spent some time talking with him about how people use the PMBOK(r) Guide. One thing that he pointed out was that it was not written as a teaching tool or learning aid. It was written as a framework to help project managers choose a project management methodology.
A lot of people ask us why there's material on the PMP exam that is not in the PMBOK(r) Guide. For example, if you do a text search through the PMBOK(r) Guide PDF (which you get with your PMI membership), you won't find any mention of "Referent Power" or "Legitimate Power" -- part of the five forms of power that you'll be tested on as part of Human Resource Management. But they definitely appear on the exam. That's because they're an important part of the modern understanding of leadership skills, so they're included as part of the PMP exam specification.
And there are other things on the PMP exam that are either absent from or not fully explained in the PMBOK(r) Guide. For example, it doesn't actually tell you how to do critical path calculations, but those are definitely on the PMP exam, and are also mentioned in the PMP exam specification.
So when we wrote "Head First PMP", we had to consult the PMP exam specification as well as the PMBOK(r) Guide to make sure that we covered all of the material that you might encounter on the PMP exam. And while the goal of the people who wrote the PMBOK(r) Guide was to create a guide, a framework, and a reference -- which, in my opinion, they did extremely well -- our goal was to create a learning tool that teaches you what you need to know in order to pass the PMP exam and become a better project manager.
Author of Head First Agile, Learning Agile, Beautiful Teams, Head First C#, Head First PMP, and Applied Software Project Management (O'Reilly)
First of all, I highly recommend "Head First PMP", as I used that book in addition to reading 4 other PMP prep books to pass easily on September 6, 2007.
Think of HFPMP in relation to the PMBOK like this:
In my development days, I would learn to program using a tutorial book such as "Introduction to C++", but later when I became more advanced and wanted to get the ultimate answer to my question, then I'd refer to the ANSI/ISO C++ Standard.
Likewise, HFPMP is a user friendly tutorial, and the PMBOK is the ultimate reference. In fact, this is one-to-one with the above analogy, since the PMBOK is an ANSI standard, and just received an ISO standard.
Below is a link to some extensive reviews of PMP prep books I used: