Bob Dylan has been rumored for years to be sitting on a treasure trove of secret materials including manuscripts, video footage, audiotape, private letters and notebooks filled with handwritten lyrics. Now it turns out that not only were the rumors true, but Dylan’s “secret archives” were more exhaustive than even hardcore Dylanologists had hoped.

The archive, consisting of about 6,000 artifacts from the singer’s private collection, has just been sold by Dylan to the George Kaiser Family Foundation and the University of Tulsa. Rolling Stone reports that the collection sold for an estimated $15 million to $20 million.

“Most of it will housed at the Gilcrease Museum in Tulsa, though access to it will largely be restricted to scholars and Dylan experts,” Rolling Stone reports. “Plans are still up in the air, but highlights from the collection will likely be displayed to the public in the near future.”

The collection includes “Dylan’s original ‘Blood on the Tracks’ notebooks, handwritten lyrics to ‘Chimes of Freedom,’ ‘Ballad of a Thin Man’ and ‘Dignity’ and footage of the artist performing at Toronto’s Massey Hall in 1980, New York’s Supper Club in 1993 and rehearsing for the 1975 Rolling Thunder Revue,” the article reports. “There’s also a manuscript of his 1971 book ‘Tarantula,’ hundreds of tape reels and a 1978 postcard from Barbra Streisand.”

The New York Times, which previewed the collection, spotlighted the “Blood on the Tracks” notebooks, reporting: “For years, Bob Dylan scholars have whispered about a tiny notebook, seen by only a few, in which the master labored over the lyrics to his classic 1975 album ‘Blood on the Tracks.’ Rolling Stone once called it ‘the Maltese Falcon of Dylanology’ for its promise as an interpretive key.”

The Times adds: “But that notebook, it turns out, is part of a trinity. Sitting in climate-controlled storage in a museum [in Tulsa] are two more ‘Blood on the Tracks’ notebooks — unknown to anyone outside of Mr. Dylan’s closest circle — whose pages of microscopic script reveal even more about how Mr. Dylan wrote some of his most famous songs.”