Oh, it's actually easy to find on https://www.rust-lang.org/learn in the second section of the page. Most likely reason for me not having found it is that the last time I looked at that page, I wasn't looking for the rustc book specifically, and that now that I wondered where rustc's user-level documentation was, the book doesn't appear when I googled simply "rustc documentation". You have to google specifically "rustc book" and that didn't occur to me, as I was expecting some kind of manpage instead.
