Effective Coaching by Myles Downey: Lessons from the Coach's Coach.

The Integral Vision by Ken Wilber: A Very Short Introduction to the Revolutionary Integral Approach to Life, God, the Universe, and Everything.

Core Coaching: Coaching for Great Performance at Work by Sheridan Maguire: A practical guide to executive coaching that provides a useful toolkit of techniques and creative approaches. 

Coaching for Performance: Growing People, Performance and Purpose by John Whitmore: The principles and practice of coaching and leadership.

The Inner Game of Tennis by Timothy Gallwey: The classic guide to the mental side of peak performance.