The Third Key — Gerald Verner

The Reverend Colin Armitage receives a parcel one morning containing a key and the intriguing message: 'This is Bluebeard's first key.' The key belongs to the cottage of a woman named Sylvia Shand, who is found there, strangled. A few days later, Bluebeard's second key arrives by post and the district nurse is found strangled in similar circumstances. The police believe a homicidal maniac is loose in the village but Armitage has other ideas. And then a third key arrives...