I can't believe I forgot one:
Opening Up: A Guide to Creating and Sustaining Open Relationships by Tristan Taormino
Edited because I can't spell, apparently
https://www.amazon.com/Opening-Up-Creating-Sustaining-Relationships-ebook/dp/B07H46BBC7
https://www.amazon.com/Ethical-Slut-Third-Practical-Relationships-ebook/dp/B01N0SA1YW