I purchased Head First PMP after a PM friend recommended me that book as a good starter guide to taking the PMP certification. As he mentioned & others have vouched, no one book including this one can help you pass the exam. What this book does is that it makes learning Project Management concepts enjoyable. This was my first book in the Head First series. I liked the book's unusual, non-linear style (with things like notes in the margins) of presentation although the excessive use of goofy pictures put me off a bit. The tone throughout is very informal, friendly & conversational which makes the guide engaging.
The 42 processes which form the core of the PMBOK (Project Management Body of Knowledge) Guide, 4th Edition and cross-cut 9 Knowledge Areas & 6 Process Groups are explained well. Processes grouped by Knowledge Area make up a chapter. Besides the 9 chapters focusing on the Knowledge Areas, there are 6 more which cover the preliminaries & practice tests with answers. Important points in them are reiterated & highlighted. 2 complete chapters including a Practice test are available for free download. You could check these out & see if the book suits you before you buy it.
On the negative side, there are some typos & errors that you would have to look out for. I found some of the examples in the book to be too unrealistic & contrived to be of practical benefit.
Summary: Whether or not you are taking the exam, I feel this is a good guide on Project Management to start with if you are an aspiring PM or one by circumstance.