NEW YORK (AP) — The authors of a new J.D. Salinger biography are claiming they have cracked one of publishing’s greatest mysteries: What “The Catcher in the Rye” novelist was working on during the last half century of his life.

Starting between 2015 and 2020, a series of posthumous Salinger releases are planned, according to “Salinger,” co-written by David Shields and Shane Salerno and scheduled to be published Sept. 3. The Associated Press obtained an early copy. Salerno’s documentary on the author opens Sept. 6. In January, it will air on PBS as an installment of “American Masters.”

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.

One of the Salinger books would center on “Catcher” protagonist Holden Caulfield and his family, including a revised version of an early, unpublished story “The Last and Best of the Peter Pans.” Other volumes would draw on Salinger’s World War II years and his immersion in Eastern religion.

A publication called “The Family Glass” would feature additional stories about the Glass family of “Franny and Zooey” and other Salinger works.

“Salinger” does not identify a prospective publisher. Spokesman Terry Adams of Little, Brown and Company, which released “Catcher” and Salinger’s three other books, declined to comment Sunday. Salinger’s son, Matt Salinger, who helps run the author’s literary estate, was not immediately available for comment.

If the books do appear, 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 canceled.

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 had been doing during his self-imposed retirement. That Salinger continued to write is well documented. Friends, neighbors and family members all reported that Salinger was writing in his final years and the author himself told The New York Times in 1974 that he wrote daily, though only for himself.

“There is a marvelous peace in not publishing,” he said at the time.

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, N.H. 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 cooperate with Salerno and Shields.

Until now, neither Salerno nor Shields has been defined by his 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 is an award-winning author whose books include the novel “Dead Languages”; a nonfiction work on pro basketball that was a National Book Critics Circle prize finalist; and “Reality Hunger,” a self-described “manifesto” for modern literature.

Their 700-page Salinger biography has new information well beyond any possible posthumous fiction. Nine years in the making and thoroughly documented, “Salinger” features many rare photographs and letters; unprecedented detail about the author’s World War II years and brief first marriage; a revelatory interview with the former teenage girl, 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 feature film.

“Salinger” both fleshes out and challenges aspects of the author’s legend. He is portrayed as deeply traumatized by his war experiences and stunned by his post-“Catcher” fame. But he also comes off as far less reclusive and detached than long believed. He does agree to the occasional interview, even initiating discussion with The New York Times, and appears sensitive to his public image. His affinity for young people is not confined to his books, and Salinger’s biographers closely track his history of intense attachments to teens, from Oona O’Neill in the 1940s to Joyce Maynard in the 1970s.

The book is structured as an oral history, featuring hundreds of new and old interviews, excerpts from newspaper accounts and previous biographies and commentary from Shields and Salerno. Those quoted range from Salinger’s children to authors Tom Wolfe and Gore Vidal to Mark David Chapman, who cited “Catcher” as a reason he murdered John Lennon in 1980.

Salerno has been promising to make headlines ever since announcing the biography and film shortly after Salinger’s death. Earlier this year, he quickly arranged lucrative deals with the Weinstein Co. for a feature film, the producers of “American Masters” 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 some $2 million of his own money and traveled throughout the U.S. and Europe in search of material. He is also adept at revealing secrets, with recent stories by The Associated Press and other media outlets, featuring photographs never previously published.

Salinger never authorized a biography, but several unauthorized books have come out over the past 30 years, notably one by Ian Hamilton. In 1987, Salinger successfully blocked release of Hamilton’s “J.D. Salinger: A Writer’s Life,” citing the use of previously unpublished letters. Hamilton described his legal battle in “Searching for J.D. Salinger,” published in 1988.