“After the Breakup: A Self-Love Journal” by Lindsey Brock

“Magnetic Partners: Discover How the Hidden Conflict That Once Attracted You to Each Other is Now Driving You Apart” by Stephen Betchen

“Mating in Captivity: Unlocking Erotic Intelligence” by Esther Perel

(also see Esther Perel's Ted Talk on The Secret to Desire in Long Term Relationships or Rethinking Infidelity)

Relationship and Sex Advice of all kinds by Dan Savage (Savage Lovecast) (Savage Love Column)

“The Power of Attachment” by Diane Poole Heller

“The State of Affairs: Rethinking Infidelity” by Esther Perel

Polyamory/ethical non monogamy

“Designer Relationships: A Guide to Happy Monogamy, Positive Polyamory, and Optimistic Open Relationships” by Mark Michaels & Patricia Johnson

“Opening Up: A Guide to Creating and Sustaining Open Relationships” by Tristan Taormino

“Polysecure: Attachment, Trauma, and Consensual Nonmonogamy” by Jessica Fern