One of the books that I most recommend is The Five Love Languages by Gary Chapman. The reason I think so highly of this piece is because of the simplicity of the idea AND the effectiveness of it.
Simple: there are five different ways (or languages) that we may express love to our partners. If we don’t speak the same language as our partner, they miss our intentions. So, this book is a translator of sorts, explaining our expressions of love to our partners.
Effective: we learn our language of love as well as our partner’s. As a result, we can better speak their language to have them feel cared for and romanced, and we can better understand when they express love in their own language, that they are reaching out to us in their way.
A great book for any couple’s library.