In our experience the following books are the best on Christian counseling.  

AACC Ethics Book:  The Ethical Code of the American Association of Christian Counselors can be downloaded for free at http://www.aacc.net/about-us/code-of-ethics/. 

Faith Therapy: Discusses the need, place, and structure of Christian counseling in the church derived from the Bible and based on the process of salvation by faith, how to overcome self-worth, significance, love and security, and how to treat common problems through faith.

Transformation: This book contains a comprehensive plan for Christian counseling derived directly from the Bible, in-depth biblical answers for complex psychological problems, models and quick reference guides for counseling 20 of the most difficult problems in the church today. 

Principles for Life:  Teaches 44 biblical principles and how to use them to construct Christian counseling plans to resolve problems not directly covered in the Bible. 

Christian Counseling: An overview of Christian counseling. 

Competent Christian Counseling:  A counseling manual by the president of AACC.

How People Grow: Analysis of the counseling process by the authors of Boundaries.

Effective Biblical Counseling:  The classic book on counseling by Dr. Crabb.

Basic Attending Skills: Discusses the basic listening skills. (Secular)

Basic Influencing Skills: A second skills book on basic influencing sikills. (Secular)

Private Practice Made Simple:  How to start your private practice.  (Secular)

The Clinical Documentation Sourcebook: Forms on CD for your private practice. (Secular)