The Mystery of Orcival. by Émile Gaboriau

A murder is discovered. The authorities quickly arrest an obvious suspect. A detective spends hours at the scene in disguise before making himself known, and proceeds to minutely examine the evidence with the assistance of a doctor, among others, before proclaiming the answer lies in a completely different direction. One would be forgiven for thinking the detective must be a certain famous Englishman and his doctor companion.
But this detective is French rather than English, a professional working for the police rather than an amateur, and indulges in candy lozenges rather than cocaine. If there is a straight line between Poe's Dupin and Doyle's Holmes, then Gaboriau's Lecoq lies right in the middle of it. He is a master of disguise, he is proud and sometimes arrogant, he notices infinitesimal things others do not, he makes great leaps in deduction while others are struggling to take small steps.
He is both strikingly similar and distinctly different than his more famous English "cousin". Although Monsieur Lecoq appeared in Gaboriau's first novel, there he played only a minor part. Here, he is the main attraction. Solving the murder of a countess and disappearance of a count requires all of Lecoq's skills, and as he steadily unravels the mystery one sees the debt that is owed by all who came after him.