The Red Widow Murders

Marie-Hortense Sanson's family - solid, kindly, bourgeois, and businesslike - went up in the world all during the eighteenth century, and in the nineties was powerful enough to shelter her English husband in the thick of revolutionary Paris, and later smuggle him out of France.
Why her descendant, Lord Mantling, kept a sealed room in his house, and why it was known as the Red Widow's Room, were things which puzzled Lord Mantling's friends, among them the great and unorthodox detective Sir Henry Merrivale.
But when murder was done in the house, it came out that murder ran in the family. Because Mrie-Hortense's family were hereditary executioners to all the high courts of France, and the Red Widow was their benefactress - Madame la Guillotine.