“Making Marriage Work” by John Gottman
This is a great introduction to some of the most important psychological research being conducted in marriage therapy. In their “Love Lab,” an inn-like setting in which couples are studied while they interact with each other, Dr. John Gottman’s team has identified (with exceptional statistical significance) a number of things that couples do or don’t do to make relationships happy or hellish. I recommend this at the start of every couples therapy.