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I'm looking for intro-to-poly book recommendations.

A friend of a friend is considering opening up their heretofore-monogamous relationship, and asked for some reading material that could provide guidance to people who are new to this sort of thing.

The recommendation I most often hear is for The Ethical Slut, which I confess I didn't finish reading, because I loaned it to a friend when I was partway through and never got it back. :) That may well still be the best recommendation, but it was published twenty years ago now, and I'm wondering if there's something else we should be pointing people to.

Back in the old days, I used to point newcomers to alt.polyamory, which I used to say had the highest wisdom-to-noise ratio of any newsgroup I had encountered; but last time I stopped by there, it was deluged with spam. (And although the FAQ includes some useful material, it was last updated in 1997.) I also used to point newcomers to local online groups in their own areas, but I don't even know if that's still a thing.

Last time I went looking for online poly resources, I felt like the ones I found were mostly flawed in various ways—giving idiosyncratic definitions of polyamory, or focusing on things that I didn't agree with, or over-proselytizing, etc. The polyamory.org main page leads off with a purity test, which I'm sure was entertaining to newsgroup regulars but I suspect is pretty offputting to newcomers.

I've been meaning for years to write my own intro-to-poly post, but the likelihood of that happening anytime soon is low.

So ... do any of you have recommendations for good reading material for people who are considering trying some flavor of polyamory? I'm not looking for y'all to post advice to newly poly people in comments here, I'm just looking for pointers to recommended reading.

I'm particularly interested in pointers to material that doesn't proselytize (in other words, that makes clear that polyamory isn't for everyone), and that gives useful practical and emotional information.