I find all of these books worth reading. Many of them are available though the library as well.
Relationship Researcher John Gottman can tell if a marriage will last within 5 minutes of observation! He has some great tools in his book and I use his work with both personal relationships and teams.
Stan Tatkin has some very practical advice as to how to keep a couple close. I love his "Couple Bubble", Rituals, and getting to know the other's "triggers".
This is a super comprehensive book with a ton to offer. Even if you use it for some of David Richo's comparison charts such as Healthy Conflict vs Stressful Drama and True Anger vs Abuse. His Five "A"s (Affection, Acceptance, Affection, Appreciation and Acceptance) make such sense and he even takes Introversion/Extroversion into account.
A really easy read and easy premise that we have different ways of expressing love and different ways we receive love. When one partner expresses in a way the other doesn't receive - well, not good. If you learn to express your love in a way that the other can receive it, it takes huge strain off a relationship. It isn't necessarily easy because we usually give the way we wish to receive and it confuses us that our overt expressions of love aren't recognized.