I've read a bunch, but the "best" ones have been:

Divorce Busting
The Divorce Remedy
John Gottman, The Seven Principles of Making Marriage Work
John Gottman, Why Marriages Succeed or Fail
Gay Hendricks, Conscious Loving
Harville Hendrix, Getting the Love You Want
Harville Hendrix, Keeping the Love yo Find
Joel Block, Broken Promises, Mended Hearts
Phil McGraw, Relationship Rescue

So far, these have helped me more than my relationship as the wife is so tight-lipped about her feelings and intentions, but they and others have been of great value in a trying time. I've learned so much, that even if my marriage fails I feel I'm a better person for having invested the time in these and other works. I feel I'm better prepared for the next relationship, should it come to that. Still, I'm not giving up hope on the current one! It's tough some days. Good luck to you.