The Relationship Cure, Gottman
Secular; Well researched; Practical and very, very helpful.
Couple Communication I: Talking Together, Miller, Nunnally, Wackman & Miller This is a revolutionary way to communicate within marriage, or any relationship.
The Constructive Use of Feelings, Dr. John Wallen
Communication: Key to Your Marriage, H. Norman Wright This updated and revised book is packed with practical advice for reducing marital conflict, managing anger, building your partner's self-esteem, and more. Ideal for engaged and married couples, as well as pastors and counselors.