Although Wilkie Collins's The Moonstone, published in 1868, and Emile Gaboriau's first Monsieur Lecoq novel L'Affaire Lerouge, released in 1866, have both been proposed as the first fictional outings for detectives, the British Library believes The Notting Hill Mystery "can truly claim to be the first modern detective novel".
Serialised between 1862 and 1863 in the magazine Once a Week, the novel was published in its entirety in 1863 but has been out of print since the turn of the century. It stars the insurance investigator Ralph Henderson, as he works to bring the sinister Baron "R___" to justice for murdering his wife to obtain a large life insurance payout. Using diary entries, letters, crime reports, witness interviews, maps and forensic evidence – "innovative techniques that would not become common features of detective fiction until the 1920s", says the British Library – Henderson's investigation slowly plays itself out, uncovering along the way an evil mesmerist, a girl kidnapped by gypsies, poisoners and three murders.
Full story at The Guardian.