<human>: So while I was at my boyfriend's house yesterday (we've been together for close to 10 months now), he asked me to get onto his laptop to change a song. A bunch of things were open, and I clicked on the page behind the spotify screen, thinking it was the browser screen. It was an ebook he had been reading. It seemed like it was talking a lot about sex, so I asked him what book it was a little while later. He said it was called "The ethical slut" and that his roommate recommended it to him, but he "didn't like it that much" (although he was on chapter seventeen). 

I let it go, not really knowing what the book was about, but out of curiosity I looked it up on Amazon today. It's about polyamory and open relationships. Now, at this point, I'm feeling a little uncomfortable with him reading a book about open relationships, which I am not interested in pursuing. It has me thinking, is this something he's interested in? Now, I'd like to bring it up to him later tonight when we talk on the phone, but I don't want to sound too accusatory or start anything unnecessary, but I need to know if this is something he's interested in or if he's just reading it because someone told him to. I'm just not sure how to bring it up in a stress-free, non-argumentative way (despite my negative feelings on the subject)?

Can you succinctly summarize the text in 1-3 sentences?
<bot>: bf reading a book on polyamory, how should I broach the subject when I'm completely against it?