This short little book helped me understand a few things about my (already happy, thank G-d) marriage that had always mystified me.
Chana Levitan asked couples about their personality differences, then studied how their ways of coping (or mishandling) these differences affected their marital happiness and duration. She eventually discovered that the most successful marriages used personality differences between spouses as a strength, rather than viewing them as a constant source of friction.
Again, I thought the book was interesting even though I am in a relatively stable relationship. I can only imagine how helpful this book would be to people with conflict in the relationship, but I suspect it would be immensely useful. Because of the narrow scope of the book, I would not recommend reading this book as the one and only book about marital relationships you read (unless you are also seeing a therapist to work on this specific issue). However, coupled with The 5 Love Languages, it's really powerful. It generated a fascinating conversation with my husband and it might improve our already close bond.
The book came to my attention because the author and the publishing house are Jewish. While there is a small number of Jewish references in this book (like, maybe 5), the book is entirely applicable and understandable to readers of all faiths.