In Unnatural Death (a.k.a. The Dawson Pedigree), Dorothy Sayers puts together a distinctive mystery story. Unlike most mystery stories, this Lord Peter novel (the third, I believe) is fairly clear from the beginning who committed the murder, if any was committed; what is puzzling is whether it can be shown that the death was, in fact, a murder, and (if so) how and why it was committed. The story is also notable for the extensive involvement of Miss Climpson. As readers of other Lord Peter novels know, Miss Climpson is a recurrent occasional character, a gossipy old spinster with a heart of gold who often helps Wimsey out. She's also noteworthy for being the only consistently religious character in the series; and one of the interesting features of Unnatural Death is Miss Climpson's difficulties in both assisting Wimsey by nosing about, and doing so in a scrupulously Christian way -- a tension she doesn't always successfully navigate. As she says, sometimes she has to get Jesuitical in a good cause. Climpson is also fairly consistently an occasion for Sayers to say something about the poor prospects of intelligent older women for finding worthwhile employment in societies like ours. Alas, there's no Dowager Duchess, but you can't have everything.
As with all of Sayers's Lord Peter novels, there are lots of allusions that are easy to miss. Bill Peschel has a good set of annotations for Part I.