The authors of a new JD Salinger biography claim they have cracked one of publishing's greatest mysteries: what the author of The Catcher in the Rye was working on during the last half century of his life.

A series of posthumous Salinger releases are planned after 2015, according to David Shields and Shane Salerno, whose book Salinger will be published on 3 September. The Associated Press obtained an early copy. Salerno's documentary on the author is scheduled to come out 6 September.

Providing by far the most detailed report of previously unreleased material, the book's authors cite "two independent and separate sources" who they say have "documented and verified" the information.

The Salinger books would revisit Catcher protagonist Holden Caulfield and draw on Salinger's World War II years and his immersion in eastern religion. The material also would feature new stories about the Glass family of Franny and Zooey and other Salinger works.

The book does not identify a prospective publisher. Spokesman Terry Adams of Little, Brown, which released Catcher and Salinger's three other books, did not immediately respond to requests for comment. Salinger's son, Matt Salinger, who helps run the author's literary estate, was not immediately available for comment.

If the books do come out, they may well not be through Little, Brown. In the mid-1990s, Salinger agreed to allow a small, Virginia-based press, Orchises, to issue his novella Hapworth 16, 1924, which first appeared in the New Yorker in 1965. But after news leaked of the planned publication, Salinger changed his mind and Hapworth was cancelled.

No Salinger book came out after the early 1960s, as the author increasingly withdrew from public life. Over the past 50 years, there has been endless and conflicting speculation over what Salinger was doing during his self-imposed retirement. That Salinger continued to write is well documented. The author himself told the New York Times in 1974 that he wrote daily, although only for himself.

But there is no consensus on what he was writing and no physical evidence of what Salinger had reportedly stashed in a safe in his home in Cornish, New Hampshire. The Salinger estate, run partly by Matt Salinger and Salinger's widow Colleen O'Neill, has remained silent on the subject since the author's death in January 2010. The two did not co-operate with Salerno and Shields.

Until now, neither Salerno nor Shields have been defined by their expertise on Salinger. Salerno is a Hollywood screenwriter whose credits include Armageddon, the Oliver Stone film Savages and a planned sequel to James Cameron's blockbuster Avatar. Shields has written 13 fiction and non-fiction books.

Their 700-page Salinger biography also features many rare photographs and letters; unprecedented detail about the author's World War II years and brief first marriage; a revelatory interview with Jean Miller, who inspired his classic story For Esme With Love and Squalor; and an account of how Salinger, who supposedly shunned Hollywood for much of his life, nearly agreed to allow Esme to be adapted into a film.

Salerno has been promising to make headlines ever since announcing the biography and film shortly after Salinger's death. Earlier this year, he arranged lucrative deals with the Weinstein Company for a feature film, PBS for TV rights and Simon & Schuster for the book. The filmmaker himself has proved as effective as Salinger at keeping a secret, with only a handful of people even knowing of the project's existence during Salinger's lifetime. Salerno spent $2m of his own money and travelled throughout the US and Europe in search of material.

Salinger never authorised a biography, but several unauthorised books have come out over the past 30 years, notably one by Ian Hamilton. In 1987, Salinger successfully blocked release of Hamilton's JD Salinger: A Writer's Life, citing the use of previously unpublished letters. Hamilton described his legal battle in "Searching for JD Salinger, published in 1988.