Another obvious choice would be The Five Love Languages. This one is critical in terms of relationships because it explains that each person receives and gives love differently.
If I may, I would like to also recommend I Don't Have to Make Everything All Better by Gary and Joy Lundberg. This is a must have book detailing the principles of validation. Very useful.
"Achieve success, but without vanity; Achieve success, but without aggression; Achieve success, but without gain; Achieve success, but without force." Lao Tzu
