NEW YORK (AP) — A 1950 Neal Cassady letter that inspired Jack Kerouac’s spontaneous style in “On the Road” is heading to auction where it’s expected to bring up to $600,000.

Christie’s is offering the 18 double-sided-page letter on June 16.

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Known as the “Joan Anderson Letter,” it was believed lost until it surfaced in 2012 in the discarded files of the now-defunct Golden Goose Press, a San Francisco publishing house where it had been placed in a “to read” pile.

Cassady wrote the 16,000-word letter to Kerouac, his fellow Beat generation figure, about an amorous relationship. Only a fragment, believed to have been copied by Kerouac himself, had been published in 1964 by John Bryan in his “Notes from Underground #1.”

Kerouac said in an interview in 1968 that Cassady’s letter “was the greatest piece of writing I ever saw” and that it gave him the idea for the “spontaneous style” of “On the Road.”

“I got the idea for the spontaneous style of ‘On the Road’ from seeing how good old Neal Cassady wrote his letters to me, all first person, fast, mad, confessional, completely serious, all detailed, with real names in his case, however (being letters),” Kerouac wrote.

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Cassady later admitted that he had been binging on Benzedrine when he wrote the letter.

In 1971, the fragment was published by City Lights as an addendum to Cassady’s book “The First Third.” It was also the basis for the 1997 film, “The Last Time I Committed Suicide,” starring Thomas Jane and Keanu Reeves.

Christie’s says title claims to the letter that were raised during a small California auction in 2014 — in which the letter was withdrawn — have been resolved, allowing the letter to be offered at its upcoming books and manuscripts sale. Christie’s declined to name the seller. However, the copyright on the letter lists Cassady family members.

The letter will tour Seattle, San Francisco, Los Angeles and New York between May 31 and June 15.

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