If you were going to read just one book on relationships I think my favorite these days is What Makes Love Last by John Gottman, PH.D. and Nan Silver. The reason I like this book is it give detailed strategies to battle critical and defensive behaviors. These two traits are like a revolving door of doom for otherwise happy couples. The couples that are detailed in the book seem, to me, more like people who love each other but hit a bump, rather than what often is the example in relationship books that tell the story of couples that have disrespect and highly dysfunctional behaviors. It also has a juicy chapter on sex which is a perk for when couples are ready to revive their passionate side but are not exactly sure how to get started.
Another favorite book is And Baby Makes Three by John M. Gottman, PH.D. and Julie Schwartz Gottman, PH.D. It is the official book from the Bringing Baby Home workshop and is great if you want to stick with content designed for new parents. It is easy to read if you are lacking sleep and have a hard time with complex language.
One last resource that is not a relationship book at all, but very helpful at getting parents are on the same page with parenting strategies, is any of the many Positive Discipline resources. Even reading a few of their blog posts will get you on the same page.
Often at the base of every relationship help book I have read it that you need to be nice. Yep. It is often that simple. Being polite and expressing gratitude at every opportunity can go along way to helping any relationship improve.