The Champdoce Mystery by Émile Gaboriau

Sequel to Caught in the Net. According to Wikipedia: Émile Gaboriau (November 9, 1832 – September 28, 1873) was a French writer, novelist, journalist, and a pioneer of detective fiction... after publishing some novels and miscellaneous writings, found his real gift in LAffaire Lerouge (1866). The book, which was Gaboriaus first detective novel, introduced an amateur detective. It also introduced a young police officer named Monsieur Lecoq, who was the hero in three of Gaboriaus later detective novels. The character of Lecoq was based on a real-life thief turned police officer, Eugène François Vidocq (1775–1857).