Murder on the Leviathan by Boris AkuninMurder on the Leviathan by Boris Akunin

Paris, 1878: Eccentric antiquarian Lord Littleby and his ten servants are found murdered in Littleby's mansion on the rue de Grenelle, and a priceless Indian shawl is missing. Police commissioner "Papa" Gauche recovers only one piece of evidence from the crime scene: a golden key shaped like a whale. Gauche soon deduces that the key is in fact a ticket of passage for the Leviathan, a gigantic steamship soon to depart Southampton on its maiden voyage to Calcutta. The murderer must be among its passengers.