The Meaning of Marriage is the best.
The popular idea of marriage as the source of happiness and fulfilment has helped to erode the institution that was created for higher purposes. The Kellers clearly and skillfully address many aspects of Christian marriage, and help their readers to look beyond a consumeristic view (I'll stay married as long as all my needs are being met) toward a God-centered view (I choose to stay committed to a person when it isn't easy because God through Christ shows that kind of grace toward me.)
Tim and Kathy address many important issues: What is love? What is marriage for? How can you reflect these purposes if you are single? What about gender differences? How are Christians supposed to view their sexuality? What about the submission/headship passages?
First of all they affirm the goodness of marriage and sex. But they quickly assert that this goodness is not just personally "good for me" (i.e. meets all my needs) but good because it builds families and societies and because it brings glory to God.
Sin and self-centeredness is what makes marriage hard. (Your own sin as much as your spouse's.) What if you began your marriage understanding its purpose as spiritual friendship for the journey to the new creation? What if you expected marriage to be about helping each other grow out of your sins and flaws into the new self God is creating? Then when you come to the [difficult] seasons, you will roll up your sleeves and get to work. (p. 149)
Romance, sex, laughter, and plain fun are the by-products of this process of sanctification, refinement, glorification. Those things are important, but they can't keep the marriage going through years and years of ordinary life. What keeps the marriage going is your commitment to your spouse's holiness. . . . Jesus died not because we were lovely, but to make us lovely. (p. 134)
It's interesting to note that the book is based on a series of sermons that Keller originally preached to his congregation of mostly singles when Keller realized that many of them were not marrying because they had wrong expectations for marriage.
This is a great book with many helpful insights. Definitely in the top ten best books I've read this year.