Spinsters in Jeopardy (Roderick Alleyn #17)

British mystery novel, first published 1953. Also known as The Bride of Death.

En route to a family vacation on the French Riviera, Inspector Roderick Alleyn glimpses from the train a shocking tableau. In a moonlit window a white-robed figure raises a knife to a woman's shadow. Thus begins his incognito exploration of the Chateau of the Silver Goat...where a jet-set cult's 'Way of Life' could spell death for a maiden lady of a certain age-and even for Alleyn's own young son unless he can unveil its illicit mysteries...