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