Two books that I really like about relationships in general are:
The Five Love Languages by Gary Chapman. It's very Christian in it's background, but still has a lot of great insights.
Living Sensationally: Understanding your Senses by Winnie Dunn. This isn't a relationship book really, but I feel it applies a lot to relationships because it talks about how our sensory processing can be different and understanding such processing differences can really help to understand our partners better.