Emile Gaboriau 
Monsieur Lecoq 

Classic French mystery novel, in English translation. 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 L’Affaire Lerouge (1866). The book, which was Gaboriau’s first detective novel, introduced an amateur detective. It also introduced a young police officer named Monsieur Lecoq, who was the hero in three of Gaboriau’s later detective novels. The character of Lecoq was based on a real-life thief turned police officer, Eugène François Vidocq (1775–1857).’ One of Robert Louis Stevenson’s favorite authors.