Context: I have worked for some 6-7 years as software developer, most of that time utilising agile methodologies (scrum, many facets of XP).
This book gives an introduction to various principles, practices and approaches that could collectively be labeled "agile". It's structured into short 2-4 page items much like "The Pragmatic Programmer" (which, like this book, is co-authored by Mr Hunt), and I've seen it sometimes touted as a great sequel to that book (in fact, I think I originally bought it because of that).
I agreed with almost every point made in the book! (Especially with items such as "Keep it Releasable", "Architects Must Write Code", "Be a Mentor", "Review Code".) At the same time, I didn't really learn much from it. If you have ever worked in a smart team doing agile development, chances are there isn't that much new for you either: most of the advice will basically sound like common sense. (Although perhaps this *could* be useful then too, for reinforcing ideas you've had or giving them a name.)
In summary, not a bad book at all, but I'd primarily recommend this only for beginner to intermediate level developers, or those who are new to agile software development. For them this can indeed be a valuable, even inspiring, quick-to-read intro.