Positive Psycholgy Coaching:  Putting the Science of Happiness to Work
Positive Psychology Coaching lays-down a good, fundamental blueprint for understanding the mindset required for moving towards coaching from a positive psychology methodology, and provides a wealth of research on the subject within the notes.  A basic understanding of positive psychology (read Martin E. P. Seligman - Flourish) should be a prerequisite to reading this book, and it should then be followed by Practicing Positive Psychology Coaching (Robert Biswas-Diener) as a means to incorporating many of the available tools to be utilized within your coaching.  Combined, these books will certainly provide significant direction for anyone looking to practice positive psychology coaching.
My church used this book for a 6-week small group study. It helped us reframe our perspective on church membership, causing us to become more active and less selfish.