The Complete Idiot's Guide to a Healthy Relationship
I'm almost sad that they called this an "Idiot's Book" because it makes it seem like the information in here is very low-level. It's actually incredibly helpful for all levels of couples - from those who have never dated anyone else before to those who have dated quite a bit but now want to learn more about their relationship.
There are chapters on everything in here - cheating on the internet, the way you deal with your partner's parents, dealing with losses, your arguing styles, how you criticize each other, working through sexual incompatabilities, you name it. I can't imagine any couple that couldn't learn a few things by reading through this. Even if one partner reads it and then talks to the other about the key topics, it'll help a relationship shake out the kinks.