The Worsted Viper (1943)
My review:
While there are some good witchcraft elements in this Norfolk tale—the Satanist Black Mass at the end in particular—it is more of a shaggy-dog thriller than a detective story proper, with venom-tipped darts shot at Mrs. Bradley from moving cars. The tale is poorly plotted, and very incoherent—the killers' motives and identities remain confused even at the end. Not one of her better jobs.