Zadig and Vathek

Two influential books that were forerunners of their genres
Zadig Zadig is a story of adventure and romance; of political intrigue and deception; of justice and fate, all gathered in one simmering philosophical broth. Although Voltaire had not intended to write a detective novel, his influence on detective literature is clear. Poe based his Dupin directly on Zadig, while Holmes's precise methods can be seen as a modern version of Zadig's own technique.
In a memorable scene near the beginning, Zadig is approached by a royal entourage who make a general inquiry to him about the disappearance of two animals. Zadig has never seen the animals and yet – using the track marks before him – he paints such a faithful picture of them as to be mistaken for the thief himself. It is the method, and not the mystery, that earned Zadig the claim of being a possible contender for the first modern detective novel.
"The first systematic detective in modern literature" – A Catalogue of Crime by Barzun and Taylor
Vathek (alternatively titled Vathek, an Arabian Tale or The History of the Caliph Vathek) by William Beckford is an eastern orientalist tale with a touch of the Arabian Nights (which had just been introduced to the west and whose exoticism had huge appeal). This novel provided the framework for the eastern adventure or orientalist genre of books that was to follow – whether adventure thriller or horror or mystic mysteries.

William Thomas Beckford (1760–1844) was an English novelist, art collector, patron of decorative art, critic, travel writer, plantation owner and a politician for some time. Beckford is remembered for a Gothic novel, Vathek (1786); for building the lost Fonthill Abbey in Wiltshire and Lansdown Tower ("Beckford's Tower") in Bath; and for his art collection.
He was reputed at one stage to be England's richest commoner. The son of William Beckford and Maria Hamilton, daughter of the Hon. George Hamilton, he served as a Member of Parliament for Wells in 1784–1790 and Hindon in 1790–1795 and 1806–1820. As a writer, Beckford is remembered for Vathek, and for his travel memoir, Italy: With Some Sketches of Spain and Portugal. He followed Vathek with two parodies of current cultural fashions, the formulaic sentimental novel, in Modern Novel Writing, or, The Elegant Enthusiast (1796) and Azemia, a satire on the Minerva Press novels, written as "Jacquetta Agneta Mariana Jenks, of Belgrove Priory in Wales"; and also published Biographical Memoirs of Extraordinary Painters (1780), a literary prank burlesquing serious biographical encyclopaedias.