Murder in the Mill-Race — E.C.R. Lorac

"Never make trouble in the village" is an unspoken law, but it's a binding law. You may know about your neighbor's sins and shortcomings, but you must never name them aloud. It'd make trouble, and small societies want to avoid trouble."

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.