The Cluny Problem. Dorothy Fielding

An innocent masked-ball party in the touristy town of Cluny turns into a puzzling scene of crime with two of the guests being found dead in a locked room! Excerpt: "Anthony!" Vivian Young made a laughing surprised clutch at a tall figure stalking ahead of her down the station platform. The man turned sharply. At the sight of his fiancée he smiled pleasantly, though a sharp observer would have said that there was something in his eyes that suggested a man about to make the best of a position not entirely to his liking. «My dear girl!» he ejaculated warmly, «what brings you to Macon? Did you get into the wrong train, or out of the right one, or what?» «I'm on my way to Cluny.»