I read This Momentary Marriage by John Piper earlier this year. It was an incredible book. Everybody should read it whether they are married or single.
at the link above you can download the book for free in pdf form. Here is the direct link to the download. It is easy to read and monumentally important to a correct Biblical understanding of marriage, sex, divorce, singleness and parenting.
Here, courtesy of Westminster Bookstore (they sell a hard back version), is John Piper talking about the book.