A young JD Salinger boasted in the early 1940s that one of his short stories would "doubtless tear the country's heart out, and return the thing a new and far richer organ".

The brag was made to a young woman in Toronto, Marjorie Sheard, with whom Salinger had been corresponding, and has come to light after the nine letters sent by the author to Sheard were sold by the now 95-year-old and her family to pay for her care. Unfortunately for the late Salinger's many fans, keen to read more from the reclusive author who published nothing between his novella Hapworth 16, 1924 in 1965 and his death in 2010, the story, "Harry Jesus", was never published. "I'll probably fail completely with it," added the writer, who was just setting out on his literary career.

The letters were acquired by the Morgan Library & Museum and shown to the New York Times. Sheard was an aspiring author who had read some of Salinger's first short stories, and got in touch asking for advice.

"Seems to me you have the instincts to avoid the usual Vassar-girl tripe," Salinger told her. "You can't go around buying Cadillacs on what the small mags pay, but that doesn't really matter, does it?"

After requesting a large photograph of his correspondent, the 22-year-old Salinger told her: "Sneaky girl. You're pretty", before adding "I sent off my last photo to a little magazine, but I'm having some more made. Rest assured, though, I'm a doll." He would sign off his letters to Sheard with pseudonyms such as "Fitzdudley", "Wormsley-Bassett" and "Flo and Benjy".

In November 1941, Salinger had had his first piece, the story "Slight Rebellion Off Madison", accepted by the New Yorker, and was awaiting its publication. He described it to Sheard as "the first Holden story" – it would appear in altered form as a chapter in The Catcher in the Rye. It had, he told his correspondent, led to his editor asking for an "entire series" on the character.

"I'll try a couple more, anyway, and if I begin to miss my mark I'll quit," he wrote. Declan Kiely, at the Morgan Museum, told the NYT that the letters were "a wonderful opening onto [Salinger's] earliest years as a writer". "He's just at the threshold of his career, but his voice is there," he said.

Later, in 1942, Salinger would tell Sheard that "God and Harold Ross [the New Yorker's founding editor] alone know what that bunch of pixies on the staff are doing with my poor script" – he was still awaiting its publication, which had been delayed by the war. "Slight Rebellion off Madison" would eventually be published in 1946.