I read this book on behalf of a family member who is going through a very difficult situation in his marriage, hoping to find some sound advice to share with him. It was excellent, and I'll definitely be recommending it.
If you're familiar with the book, _Sacred Marriage_ by Gary Thomas, this book is very similar (but goes into more depth regarding marriages in crisis).
The advice is sound and biblical. Like Thomas (whom she often quotes), Vernick emphasizes that marriage is an opportunity to pursue holiness, not just happiness. She reminds us that we should keep an eternal perspective - not looking for an easy out that will make us "happy" for a few decades, but rather persevering and succeeding in our commitment to pursue godliness, which will result in eternal rewards. Responding to sin in your spouse in a biblical manner will result in the satifaction of knowing you have done the right thing and that it pleases God, and this is the motivation to hang in there when the proverbial going gets tough. She also emphasizes the need for "tough love" or allowing certain consequences to happen to your spouse in order to bring about repentance/change and not to simply enable them to keep sinning.
While this book is certainly directed towards marriages that are in a major crisis (adultery, addictions, abuse, etc.), I think this book would be helpful for any married couple to read. The principles certainly apply to the day-to-day "normal" conflicts/irritations within a marriage, and I think every Christian couple would benefit from reading it.