#20: The Alan Turing Notebook (1942)
Turing wrote this 56-page notebook at Bletchley Park while working to break the German Enigma code, filling it with complex mathematical notations. After his death in 1954, it passed to his friend Robin Gandy, who used its blank pages to record his own dreams at a psychiatrist’s suggestion. Sold at Bonhams New York in April 2015 for $1,025,000, it is the only extensive handwritten Turing manuscript known to survive.
