In our experience the following books are the best on Christian life coaching.  

Christian Coaching:  Gives an oaverview of the field of Christian Life Coaching.

Christian Life Coaching Handbook:  By one of the top coaches in the U.S

Leadership Coaching:  One of the best books on coaching skills.

The Business and Practice of Coaching:  How to get started (Secular)

The Call Journey:  The map of how God works with people to achieve their calling.

Believe That You Can:  Finding your dream and call in God.