Death in High Heels - Christianna Brand

Inspector Charlesworth investigates a strange murder in a dress shop
The sales room at Christophe et Cie is staffed by five young women. Each is beautiful in her own way-and each could be a murderer. One morning, two of the women purchase some oxalic acid to clean a stain off a Panama hat. No one knows how the poison gets into Miss Doon's system, but it doesn't take long to kill her. When Inspector Charlesworth steps into the little shop, he finds a dozen motives and no clear solution. Everyone in the shop was jealous of Miss Doon, for as the owner's girlfriend she was the favorite to head up the store's new Riviera branch. Romantic feelings for his chief suspect sidetrack Charlesworth, and it takes a second murder to put him back on the trail of the killer.