Murder in the Mill-Race [Speak Justly of the Dead] by E.C.R. Lorac

When Dr Raymond Ferens moves to a practice at Milham in the Moor in North Devon, he and his wife are enchanted with the beautiful hilltop village lying so close to moor and sky. At first, they see only its charm, but soon they begin to uncover its secrets—envy, hatred, and malice.

Everyone says that Sister Monica, warden of a children's home, is a saint—but is she? A few months after the Ferens' arrival her body is found drowned in the mill-race. Chief Inspector Macdonald faces one of his most difficult cases in a village determined not to betray its dark secrets to a stranger.