Neal Cassady wrote 16,000 word letter over three days in December 1950.
Later admitted to having been on amphetamine Benzadrine the whole time.
Kerouac said the 18-page note was 'the greatest piece of writing I ever saw.'
Letter was lost for 60 years but was found in a defunct publishing house
Neal Cassady's letter to Jack Kerouac which inspired On The Road goes up for sale | Daily Mail Online