Douglas J. Brouwer: Beyond "I Do": What Christians Believe About Marriage
This is one of the best books about what a marriage is. It's really a theology of marriage rather than a "how to" manual, but it's perspective provides ample context for why the How-To stuff works.
Gary Chapman: The Five Love Languages: How to Express Heartfelt Commitment to Your Mate
This instant classic states clearly what we know intuitively: We all need love, but we don't all love the same way. Some need words, some need touch, some need time together. This is a must read for lovers and parents!
Harville Hendrix: Getting the Love You Want: a Guide for Couples
Despite the self-centered title, this book is a very helpful, high-level look at how marriages work. Why do honeymooners fall in love? Why do they fall out of love? This book has some answers!
Rob Bell: Sex God: Exploring the Endless Connections between Sexuality and Spirituality
Rob Bell brings his usual lyrical and readable style to a Biblical discussion of human sexuality as a gift of God.
Tim & Beverly LaHaye: the Act of Marriage
A 1976 classic on the beauty of sexual love. Grounded in a biblical perspective, this explicit book has served many a guide to the sexual aspect of their marriage.
Lewis B. Smedes: Sex for Christians: The Limits and Liberties of Sexual Living
The venerable Lewis Smedes offers accessible and practical perspectives on how God created sex to be a part of our lives.
Dennis Rainey: Moments Together for Growing Closer to God (Moments Together)
This devotional is an excellent place to being forming a spiritual life together. Just be sure you have a little time, since the questions often lead to great discussions!
Tim Kimmel: Little House on the Freeway: Help for the Hurried Home
Feel like you have more life than you can handle? Feeling Harried, Harassed, and Hurried? This book is for you!
Scott M. Stanley: A Lasting Promise: A Christian Guide to Fighting for Your Marriage
Dennis Rainey: Rekindling the Romance: Loving the Love of Your Life