Here's another book, reviewed here earlier this year, that should be noted as part of the Vintage Mystery Reading Challenge over on the Bev's Readers' Block blog.
First published in 1944, "The Case of the Gilded Fly," by Edmund Crispin, was the first mystery to feature the sleuthing talents of Oxford University Professor Gervase Fen. It is the story of murder among a theatrical troupe - an apparently impossible murder, in a locked room. The victim apparently was shot at point-blank range...but witnesses are prepared to swear that nobody but the victim was in the room. Yet suicide was equally impossible, not least because there's no gun in the room. As one of the police officers observes, rather gloomily, Fen proves pretty clearly that it couldn't have been suicide, while other witnesses prove that it couldn't have been murder, so the only obvious solution must be that it never happened in the first place.
As with all of Crispin's novels, "The Case of the Gilded Fly" has some first-rate touches of humor, as well as some truly horrifying scenes. The writing is wonderful, the characters are entertaining and distinctive, and Professor Fen is in his glory.
I am entering this in the Vintage Mystery Reading Challenge in category 16, Locked Rooms. I do recommend checking out some of the other books that are being read and reviewed by other participants in the challenge - or, perhaps, you'd like to join in the challenge yourself? You'll find details at the link.