Gaboriau's first book, L'Affaire Lerouge (1866), is his best known work. It was inspired by a famous murder. In it, a wealthy widow is found brutally murdered in her palatial mansion. A judge, assisted by two policemen study the murder scene, then the judge calls in a wise old amateur detective to help with the case. L'Affaire Lerouge was first published in the newspaper The Age and became a big hit.
Interestingly, one of the policemen, a man known as Monsieur LeCoq (Mister Rooster), becomes the hero in three of Gaboriau's later novels. The character LeCoq was inspired by Eugene Francois Vidocq, a French criminal who became the first private detective and began the discipline of criminology. Vidocq also inspired Victor Hugo, Honoré Balzac and Alexandre Dumas.
Gaboriau also introduced the convention of a wise sidekick who helps the hero figure out the mystery. His character was Jean Tabaret, whose nickname was Pére Tireauclair (Father Bringer of Light). From the comfort of his bed, the old man uses deductive reasoning to help solve the crime.
Gaboriau admired the writing of Edgar Allan Poe, especially his short story The Murders in the Rue Morgue, which was first published in 1841 and is considered the first detective story.
Among Gaboriau's other novels are Baron Trigault's Vengance (1870), The Count's Millions (1870), Within an Inch of His Life (1873), and Other People's Money (1874).