Dead Water by Ngaio Marsh

A quirky Roderick Alleyn mystery about faith, greed – and murder. Times are good in the Cornish village of Portcarrow, as hundreds of unfortunates flock to taste the miraculous waters of Pixie Falls. Then Miss Emily Pride inherits the celebrated land on which Portcarrow stands and wants to put an end to the villagers' thriving trade in miracle cures, especially Miss Elspeth Costs's gift shop. But someone puts an end to Miss Cost herself, and Miss Pride's guardian angel, Superintendent Roderick Alleyn, finds himself on the spot in both senses of the word...