The second Jill Paton Walsh-completed Lord Peter Wimsey book. In this case Walsh has had rather more of a hand in the plotting than she did in "Thrones, Dominations": this is based on "The Wimsey Papers", a collection of letters between characters in the Wimsey stories that Sayers wrote for "The Spectator" during the Second World War. (They aren't in the collection of Sayers short stories I've got, but you can find them online fairly easily.)
Since the letters weren't originally intended as a novel, the rest of this -- including the all-important murder mystery -- has been fabricated entirely by Walsh. It's very much a Harriet book rather than a Peter one, and there's a lot more background and character development than actual mystery-solving; I feel this is for the best, since the original Sayers material consists entirely of background! The overall effect is pleasing; it kept me interested all the way through, and I completely failed to predict the ending -- as usual.
The one detail that bugged me was the throwaway reference to Bletchley Park; it's perfectly accurate, but there's no way that Sayers could have written it at the time!
Recommended. I wish Walsh would write more, but that seems a bit unlikely now...