In our experience the following books are the best on Christian life coaching.
Christian Coaching: Gives an overview 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.
The Dream Giver: A classic analogy of the struggles faced in pursuing ones dream.