The Lerouge Case. by Émile Gaboriau

Considered by many to be the first detective novel, The Lerouge Case (aka The Widow Lerouge) introduces Monsieur Lecoq (later Inspector Lecoq), a former "habitual criminal" who becomes a police officer. Emile Gaboriau based Lecoq at least in part on an actual criminal-turned-police-officer, Eugène Vidocq, who went on to be the first director of the Sûreté. In this first book, Lecoq plays a relatively small part, the bulk of the mystery solving being done by Lecoq's mentor Tabaret, an amateur detective.
Gaboriau thus introduces both a police detective and an amateur detective at the same time. Many of the attributes now taken for granted in the mystery arena originated with Gaboriau and Lecoq-hyper attention to detail, mastery of disguises, amateur "agents" who assist the detective, and the above-mentioned amateur detectives that assist and sometimes out-perform the police versions. Gaboriau's Lecoq novels were wildly successful until another amateur detective named Holmes made his appearance.
Holmes even comments on Lecoq in A Study in Scarlet, dismissing him as a "miserable bungler" in response to Dr. Watson's question. Nevertheless, Arthur Conan Doyle was obviously influenced by Gaboriau and Lecoq, as many of Holmes' traits can be seen first in Lecoq.