If you want to truly understand the CMMI, its benefits, and a frank discussion of the challenges you'll encounter when implementing it, this book is worth its weight in gold.
First, it distills all of the information you can freely download from SEI (see ASIN B0000Y4PCA) into a more manageable volume that is sequenced for understanding both the CMMI and how to implement it. While the SEI documentation is complete and comprehensive, it is also information overload.
Second, this book does not conflict with SEI's documentation. In addition to being consistent with it, it also provides assessment and implementation techniques that SEI developed and recommends, such as the Standard CMMI Appraisal Method for Process Improvement (SCAMPI).
Third, this book comes with a CD ROM with checklists and templates that will greatly aid in implementation.
The book is structured into three parts - (1) an introduction, which is fairly generic, (2) a detailed description of CMMI process areas for levels 2-5, and (3) an implementation approach that is complete and realistic. I like the fact that the challenges of implementing CMMI are covered, as well as a sensible discussion of whether the CMMI is right for your organization. This is a nice contrast with books that proclaim the CMMI to be absolutely necessary without regard to business and organizational realities.
Material in the 17 appendices add further value to this book. In addition to the checklists and templates, this collected information also discusses metrics and related topics that did not merit a chapter in the body of the book. If you are implementing the CMMI you'll find this book to be a perfect resource in the discovery and elaboration phases of the project, and an excellent guide during the implementation.
