The Five Love Languages is a good book that anyone in a relationship should read. My wife and I both think our relationship is good, and yet we still both chose to read it.
It's better to figure out all the things that work in a relationship and to nurture those early than it is to try to repair a broken relationship later. I like the car metaphor with this. A car will run for a long time with standard maintenance, but if you never change the oil or rotate the tires or wash it, then in a fairly short time the car will fall apart and not be fixable. Relationships can be the same.
The Five Love Languages helps you to understand what makes your spouse or loved one tick. Maybe you were naturally doing something early in the relationship during the "in love" phase that you aren't anymore. This book will help you to recognize those things that mean the most to your relationship so you can nurture it and help to remain strong.