Love and Respect - Eggerichs
This book based on Ephesians 5 is a great book which irons out specific ways you can better understand you spouse and enrich your marriage.
The Five Love Languages - Chapman
I have found on many occasions during the course of marriage therapy that couples have been expressing their love to one another in a way that was not understood by their partner. Chapman explains this in a way which makes sense and can help individuals understand their married partner in a way they never have.
Covenant Marriage: Building Communication & Intimacy - Chapman
Another wonderful book from Chapman which describes how you can have better communication and intimacy in marriage
The Meaning of Marriage - Keller & Keller
This book offers instruction on how to have a successful marriage and to know God and love more deeply as a person.