"Emotional Infidelity: How to Affair-Proof Your Marriage" by Gary Neuman was one that helped us a lot.
Granted, I was a little put off by the title at first pass. I don't think that anything is 100% disaster proof, but his advice and message helped us gain better perspective.
If you're looking for personal healing, I highly recommend anything by Pema Chodron. "When Things Fall Apart" is an excellent start.