Coaching Agile Teams: A Companion for ScrumMasters, Agile Coaches, and Project Managers in Transition (Addison-Wesley Signature Series (Cohn), 2010)

This is the book that I personally use as my Agile coaching reference.  Lyssa has established herself as the Agile Coaches coach and if you want to embrace the coaching aspect of being a Scrum Master or Agile Coach, this is absolutely a must read.

Agile Retrospectives: Making Good Teams Great (Pragmatic Programmers, 2006)

This book has been the go to book for Agile Retrospectives since its release.  It is an easy read and quickly gives you the foundation for effective retrospectives.  The book is a great reference as your team progresses and encounters new challenges.