I haven’t chosen to include everything I’ve ever read on these topics, just the most recent and relevant books.
More Than Two: A practical guide to ethical polyamory by Franklin Veaux and Eve Rickert is the most recent book I’ve read, and the most important, on relationships. I think that the skills covered are vital for anyone engaging in any type of intimate relationship, be it monogamous or polyamorous. I’m a bit biased regarding this one, as I was a Kickstarter supporter before the book was even released—but I think it has more than lived up to its promise.
Opening Up: A Guide to Creating and Sustaining Open Relationships by Tristan Taormino is a great book on open relationships. I found it to be much better than I had expected. The appendices, in particular, are full of valuable information.
Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships by Christopher Ryan and Cacilda Jethá is one of the best books I’ve read on the anthropology of relationships.