Israel's National Library, which has a Zweig archive comprised of letters and manuscripts, was recently contacted by 90-year-old Hannah Jacobson from Bat Yam, a coastal city in central Israel.
Jacobson had in her possession 26 letters and six postcards sent by Zweig, born on November 28, 1881, to her late stepfather Hans Rosenkranz over a 12-year correspondence.
The correspondence began in 1921 when Rosenkranz was 16 and Zweig was at a high point in his literary career, which eventually included works such as the novellas "Letter from an Unknown Woman" and "Amok" as well as his memoir "The World of Yesterday".
The academic world has had almost "no access" to the Zweig letters to Rosenkranz until now, said Stefan Litt, the German-born archivist in charge of the Zweig collections.
"This is really a brand new finding, not just for researchers but for the whole public," Litt told AFP.
The letters start with Zweig offering Rosenkranz advice on how to face life as a young Jewish man in Germany, Litt said.
"The Jew must be proud of his Judaism and glorify in it," Zweig wrote, warning Rosenkranz however to "not brag about accomplishments" of the Jewish people.
Zweig was a pacifist who considered himself a citizen of the world and not a Zionist, or those who have worked toward the re-establishment of a Jewish homeland.
In another letter to Rosenkranz, he discouraged the young man from immigrating to British-mandate Palestine, where the state of Israel was later established in 1948, but told him to "learn languages". "Who knows, maybe Germany and Europe will become so stifling that the free spirit will not be able to breathe within them," Zweig wrote.
At the same time, Zweig greatly admired Zionism visionary Theodor Herzl and praises the Austrian-Hungarian journalist in the third letter of the series to Rosenkranz.
Zweig had reason to admire Herzl for more than his politics, according to Litt. "Herzl was actually the one who enabled him (Zweig) to publish literary writings in the Neue Freie Presse," the newspaper where Herzl served as literary editor, said Litt.
Beyond the content of the letters, their very existence says a lot about Zweig, according to Litt.
"What is striking here is that Zweig took his time to respond to that young guy, who was after all just 16 years old when he wrote to Zweig," the archivist said.
The two met twice over their years of correspondence.
The letters evolve over the years to Zweig giving professional advice to Rosenkranz, who became a publisher in Germany before marrying and eventually immigrating to Palestine in 1933.
He fought in World War II, divorced, changed his name to Chai Ataron and worked as a journalist with Israeli newspaper Haaretz and the Jerusalem Post, before committing suicide in 1956.
As for Zweig, he would flee Europe after Hitler rose to power, eventually winding up in Brazil, where he committed suicide along with his wife in 1942. He died childless, shortly after finishing his memoir.
To Litt, the find is exciting not only to researchers but to the general public as well, with Zweig "experiencing right now a sort of renaissance".
US director Wes Anderson partly based his 2014 film "The Grand Budapest Hotel" on Zweig's writings. This year's film "Stefan Zweig: Farewell to Europe" depicts the writer's time in exile.
The National Library is set to digitise the documents soon and upload them to its website, where they will be accessible to the public, said Litt.
Could there be other Zweig letters waiting to be discovered? "I hope so!" he said.