Marquis de Sade's manuscripts (partially found literature; 18th-19th century)
Marquis de Sade is one of the most controversial writers of all time. His writings were highly sexual and showed a heavy amount of sadism. He was imprisoned several times during his life and was eventually thrown into an insane asylum. Once imprisoned, he would continue to write his disturbing works. When prison guards confiscated every piece of writing material they could from his room, it is rumored that he would take sharp objects and start carving his writings into the walls, and when that was taken from him, he would bite into his fingers and write in blood. He spent 32 years of his life imprisoned for his "perversions".
After de Sade's death, his son burned many of his unpublished manuscripts out of shame. Many of de Sade's works were lost forever as a result. However, a few materials pop up every now and then. In 1905 one of his most disturbing writings was found titled The 120 Days Of Sodom. It was found in a small scroll of paper inside one of his cells, somehow escaping looters for decades. De Sade said he had "cried tears of blood" over its loss. The manuscript can easily be found online. If you wish to pursue it, be warned: it is considered one of the most depraved, depressing, and disgusting books ever written.
De Sade has slowly been vindicated, being recognized as a very disturbing, yet intellectual and revolutionary author. Many of his lost works are still sought after by historians, collectors, and scholars alike. Many manuscripts were for finished works, and were only known to have existed via journal writings of his.