Thanks to the ACM talk (which I’m still in the middle of listening to), when I got the .io welcome message, I knew exactly where to find the “bookmark” button – (not!) obviously under a “…” additional options menu 
. Which, oddly, disappeared when I actually used the bookmark button, which replaced it.
I expect further awareness-raising after I get to the book itself!
I hadn’t noticed that – thanks! The Bookmark concept in Discourse seems to be quite rich, related to the Snooze concept used by some email clients. The UI switch you’re referring to seems to be just a temporary display change, because if you reload the page you get the ellipses back (but, helpfully, if you bookmarked the post, the bookmark icon appears separately as an option).