I had been in a relationship for a few months when I first started reading this book. Since the beginning, I could read things that were relevant to me and my relationship. Even about a week after I started reading the book, my partner said something along the lines of: "Umm... I should probably start reading that book of yours, you seem to have gained all this insight into things."
The book is written in a very clear and logical way. It's simple to read, yet it touches very big issues we have and face that we are not even aware of. It makes us realize that there are things we might 'know' in theory, but when we are in a relationship we don't quite think about. This book puts everything into perspective and makes it easy for us to realize in what parts we need to improve, and stop just pointing out what's wrong with the other person.
Definitely read it, I think everyone would get something out of it. Worst case scenario, it'll be a refresher of things you may know. We all need one of those every now and then. :)