The Gottman Institute
The Gottmans have been researching marriage and relationship for over 40 years. They have tons of articles on all things marriage, mental health, and relationships.
A Celebration of Sex
Doug Rosenau covers so much in this book! Arousal, erogenous zones, sex after kids and in menopause, sexual dysfunctions, different sexual positions, etc. It’s a great gift for newlyweds to introduce them to healthy sexuality and give them something to go back to in different life stages.
A List of Feeling Words
Psych page has a great list of words describing all sorts of emotions. It can be helpful to go through lists like these as you’re learning to name what you’re feeling and just list anything that seems to fit.
Boundaries
There are few books that I recommend as frequently as I recommend this book. A key element of healthy relationships and being able to understand, communicate and respect healthy boundaries.