I echo Yves' statement in that I wish I had this book when I started out coaching teams. The book covers all the bases to get started as a coach.
For coaches like me who have a few years' experience, some sections might seem dry, but overall you will gain insights in many areas. The PrOpER (Problem, Options, Evaluate, Review) model for deciding what to do in a situation is an example of something I learned. Overall, I feel that Agile Coaching helps me go to a deeper level on the practices I am already using, as well as teaching me about technical practices less familiar.
The book is an easy read, organized so you can dive into the topics that matter most to you now and can be used as a quick reference later.
My road to learning how to be a more effective coach involved scouring posts and articles online, discussing in mailing lists and talking to other coaches. Agile Coaching is a one-stop shop, packed with practical advice, examples to work from and stories from the trenches.