Enter Lecoq and Tabaret
Emile Gaboriau created master detective Monsieur Lecoq, who would appear in a number of books. In The Widow Lerouge (L'Affaire Lerouge), however, Lecoq was a young man on the police force, and it is his senior and mentor Pere Tabaret (also known as Tirauclair) who is centre stage.
The book begins with the murder of the Widow Lerouge and young inspector Lecoq is on the scene straightaway. The complexity of the case however forces Lecoq to invite detectives Tabaret and Daburon to help and they bring their own methods to the case.
*This book has the distinction of being credited (arguably) as the first detective novel in the sense that it featured a central detective as well as employed themes of detection. Chronology wise it sat after Poe's Dupin and before Sherlock Holmes and alongside Wilkie Collins (all available as yellowbacks) whose The Moonstone is credited with having enough elements to be classified as a detective novel (the first in the English language).