The Five Love Languages: How to Express Heartfelt Commitment to Your Mate by Gary Chapman
John Gottman, PhD, has several books based on his research in his “love lab” at the University of Washington. All of his books are excellent, these especially:
The Seven Principles for Making Marriage Work: A Practical Guide from the Country’s Foremost Relationship Expert by John Gottman PhD, and Nan Silver
The Relationship Cure: A 5 Step Guide to Strengthening Your Marriage, Family, and Friendships by John Gottman
Five Days to a New Marriage by Terry Hargrave, Ph.D. and Shawn Stoever, Ph.D.
Getting the Love You Want: A Guide for Couples by Hendrix Ph.D., Harville and Hunt PhD, Helen LaKelly
The DNA of Relationships by Gary Smalley (he has several books—all good information)
His Needs, Her Needs: Building an Affair-Proof Marriage by William F. Harley, Jr.
Fighting for Your Marriage by Howard J. Markman, Scott M. Stanley, Susan L. Blumberg
Start, Love, Repeat: How to Stay in Love with Your Entrepreneur in a Crazy Start-up World by Dorcas Cheng-Tozun