When I heard that Misha Collins's wife literally wrote the handbook on threesomes, I had to check it out for pure curiosity and fangirl's sake. Sadly, my adventures in that realm are purely academic/vicarious...but I did get some tips on how to try to get my husband on board, plus good reminders not to push or wheedle.
I'm a polyamorist at heart despite being in a monogamous marriage, so perhaps this is preaching to the choir in my case. I liked the book. Mostly. She covers ideas like introducing the idea to one's partner, dating strategies, communication techniques, self-awareness, safer sex, deepening relationships and how to manage those (not just sex!), embracing or exploring one's gayness or bisexuality, coming out as 'trisexual', and even a position guide and an appendix with further resources. The book is very thorough.
It has a friendly, conversational vibe, though she does get a bit twee for my tastes. If I never see the word “schtooping” again, I will die happy regardless of whether I ever actually get my threesome on. There are also quite a few typos that really should have been caught on edit, but otherwise it's a really great resource and interesting read.