Men and Women: Enjoying the Difference (1991/2013) by Larry Crabb sets out to explore gender differences, particularly within the marital relationship. Having said that, I believe that a substantial part of this book would be useful to anyone, married or not.
Men and Women is a practical exploration of the notion of relational sin, and particularly our tendency to be self-centered. Crabb argues that when things go wrong in relationships, the primary reason is that people are committed first of all to themselves. On page 23, he wrote "it is the act of putting ourselves at the center of the universe, where God belongs, that is unqualified sin." That thesis drives the remainder of the book.
Having done his homework, Crabb explores the two typical "camps" of understanding biblical marriage, what he terms the egalitarian and the traditionalist. He believes that problems ultimately emerge from both of these traditional understandings, on the one hand suffocating legalism and on the other self-expressing freedom. the problem with both of them is that they are self-, rather than other-, centered. In typical Crabb fashion, he describes things in a fresh way that others don't always grasp while remaining firmly committed to Scripture.
Men and Women is a great book for the health of marriages, but I would say that it is also an excellent book for relationships in general. I am committed to the notion of self-denial as essential Christian teaching and this book gets right to that heart.