This is book 18 of the "Lord Peter Wimsey" series.

This note contains spoilers for the book.

Continuation of the work of Dorothy Sayers. Lord Peter tells Harriet about his first case (with considerable help from Bunter), which then comes back to life, turns out to involve up to 4 murders, and eventually gets solved.

Along the way we get to see their and Bunter and Hope's children rescuing heirlooms from the wreckage of Duke's Denver, and Helen moves into the Dower House. Hey, I said there were spoilers.

There's a nicely baroque plot, involving two very similar super-fancy huge carved emeralds—no, wait, there are three—, and a generational revenge plot even. The emeralds can be told apart by professional experts, or by anybody who can read Persion written in Arabic script (the backs turned out to be engraved with three adjacent lines from a famous poem). I would think anybody with a decent photo of the backs could tell them apart, and Bunter even takes one such photo in the early days of the mystery (as related here). This is set in about 1950, so photos aren't that hard, although Bunter's Leica is singularly ill-suited for photographing gemstones (can't focus close enough). Still, SLRs weren't by any means dominant yet then.

And Peter gets to demonstrate in considerable detail, again, that when you know how you know who.