My first book rec is Mapping the Terrain of the Heart by Stephen Goldbart and David Wallin- a book I refer to often in my psychotherapy practice. It offers our emotional world as an inner terrain to get to know and it also goes through the different capacities we need to build to have longstanding, deep romantic relationships. Not a self help book, not an academic work but somewhere in between.
If you are someone who wants to grow individually as well as relationally, this is a lovely book to check out.