Nine Coaches Waiting by Mary Stewart. Originally published in 1958, From Wikipedia: “The novel tells the haunting tale of a young English governess, Linda Martin, who travels to the Château Valmy, near Thonon-les-Bains, France, to take care of nine-year-old Philippe de Valmy. There she finds herself tangled in a plot to murder her charge and tries to save him, which eventually results in the revelation of a dark secret.”
