According to California auction house Profiles in History, who are auctioning the letter, it was never lost, but rather misplaced.
Poet Allen Ginsberg had tried to get the letter published and mailed it to Golden Goose Press in San Francisco. There it remained, unopened, until the small publishing house folded. The owner planned to trash every unopened submission he still had in his files. However, the operator of a small, independent music label who shared an office with publisher took every manuscript, letter and receipt in the Golden Goose Archives home with him.
The note was an 18-page rambling stream of consciousness to his friend in 1950. Here's an excerpt of a letter that started the fuss.
I am sitting in a bar on Market St. I'm drunk, well, not quite, but I soon will be. I am here for 2 reasons; I must wait 5 hours for the bus to Denver & lastly but, most importantly, I'm here (drinking) because, of course, because of a woman & what a woman! To be chronological about it:
I was sitting on the bus when it took on more passengers at Indianapolis, Indiana – a perfectly proportioned beautiful, intellectual, passionate, personification of Venus De Milo asked me if the seat beside me was taken!!! I gulped, (I'm drunk) gargled & stammered NO! (Paradox of expression, after all, how can one stammer No!!?) She sat – I sweated – She started to speak, I knew it would be generalities, so to tempt her I remained silent.
She (her name Patricia) got on the bus at 8 PM (Dark!) I didn't speak until 10 PM – in the intervening 2 hours I not only of course, determined to make her, but, how to DO IT.
I naturally can't quote the conversation verbally, however, I shall attempt to give you the gist of it from 10 PM to 2 AM.
Without the slightest preliminaries of objective remarks (what's your name? where are you going? etc.) I plunged into a completely knowing, completely subjective, personal & so to speak "penetrating her core" way of speech; to be shorter (since I'm getting unable to write) by 2 AM I had her swearing eternal love, complete subjectivity to me & immediate satisfaction. ...
He started working on the first of several versions of the novel as early as 1948, based on experiences during his first long road trip in 1947. However, he remained dissatisfied with the novel. Inspired by a thousand-word rambling letter from his friend Neal Cassady, Kerouac in 1950 outlined the "Essentials of Spontaneous Prose" and decided to tell the story of his years on the road with Cassady as if writing a letter to a friend in a form that reflected the improvisational fluidity of jazz. In a letter to a student in 1961, Kerouac wrote: "Dean and I were embarked on a journey through post-Whitman America to FIND that America and to FIND the inherent goodness in American man. It was really a story about 2 Catholic buddies roaming the country in search of God. And we found him."
Nowadays, novices are advised to study the writing of great authors and try to emulate them. I guess Kerouac did nothing more than that. He did the work while using his friend's style.
Is this any different than following a great leader, or sharing inspiring quotes?