This is book 12 of the "Lord Peter Wimsey" series.

This note contains spoilers for the book.

I'd been reading too much new stuff, so it was time to grab an old friend, and this one won.

Somehow I noticed the last few pages in more detail this time. And I finally found a web page that explained Peter's proposal and Harriet's acceptance in English, even. I wasn't even sure for quite a while whether that really was the proposal and acceptance. Well, yes, it was, but the idiom was extremely bizarre -- reference in Latin to Oxford tradition in degree granting. Which suits the two of them and the theme of the book well enough, but obviously I'm not part of Sayers' intended audience, not reading any of Greek, Latin, or French very well.