The Moving Finger — Agatha Christie

The placid village of Lymstock seemed the perfect place for Jerry Burton to recuperate from his accident under the care of his sister, Joanna. Then the poison pen letter arrived, viciously alleging illicit sexual activity between Jerry and Joanna. Even more shocking, obscene mail had been turning up all over the village. When a suicide results, the town is thrown into shock. The vicar, the doctor, the servants are all on the verge of accusing one another, when help arrives from an unexpected quarter. The vicar's houseguest is none other than the spinster detective, Miss Jane Marple.