He claimed to have taught himself to read and write before his first year at school, to have started serious fiction writing aged 11, and to have known for certain that he would become a professional writer when he was only 15.

Thirty years after Truman Capote’s death, the rest of the world can now make up its own mind about his work as a precocious boyhood genius.

On ThursdayToday, previously unknown stories by the author of In Cold Blood and Breakfast at Tiffany’s are being published for the first time after being rediscovered by his German publisher earlier this summer.

Four of the stories, written when Capote was aged between 14 and 17, appear in today’s edition of the German weekly newspaper Die Zeit, while Random House will publish a collection with a further 17 short stories in 2015.

Some of the stories had originally appeared in school magazines, others have never before been published.

Peter Haag, owner of Zurich-based publisher Kein & Aber, and journalist Anuschka Roshani, discovered the stories in the Capote collection at New York’s Public Library, where their manuscripts had previously escaped the attention of archivists and academics.

Originally, the pair had set out to hunt down the lost first chapter of Capote’s tell-all novel Answered Prayers. Instead, they unearthed works which, they and other Capote experts claim, show a writer much older than his years.

“When he was 23, he used to joke that he looked like he was 12,” Roshani told Die Zeit, “but when he was 12 he wrote like others did aged 40.”

Gerald Clarke, an American contemporary of the author whose biography, Capote, was turned into an Oscar-winning film starring Philip Seymour Hoffman in 2005, said: “These stories are, by definition, juvenilia, and they are not as mature as the stories that made him famous a few years later, at the ripe old age of 21 and 22.

“But even at 15 he had mastered the storyteller’s art of creating suspense and building character. He makes the reader want to turn the page and go on, which I can’t say about a lot of more experienced writers, then or now.”

Capote may nowadays be most famous for having helped to invent the “New Journalism” of the 1960s and 1970s, but it was his short stories that first gained him a reputation in the US, having pieces published in Harper’s Bazaar and the New Yorker in his late teens.

In a 1957 Paris Review article, Capote told interviewer Pati Hill that his “unswerving ambition” still revolved around the form of the short story.

“When seriously explored, the short story seems to me the most difficult and disciplining form of prose writing extant,” he was quoted as saying.

Many of the previously unpublished stories from his teenage years concern outsider figures. Miss Belle Rankin, first published in the Greenwich school magazine The Green Witch in 1941, tells the story of a lonely, ageing dame from the deep south, mocked by her fellow villagers.

Another story, Saturday Night, is narrated from the perspective of a black man who takes another man’s girlfriend out on a date to a jazz club – a milieu which a young boy growing up in racially segregated Alabama is unlikely to have had first-hand knowledge of.

Some of the stories suggest an autobiographical dimension. In This One is for Jamie, Capote, whose parents divorced when he was four, tells the story of a young boy who is neglected by both his parents and his nanny. On the playground, he strikes up an unlikely friendship with the mother of another boy, who is too ill to come out an play: each helps the other to compensate for an absence in their family life.