The Old Weird America: The World of Bob Dylan's Basement Tapes, 2nd edition, by Greil Marcus, (previously published as The Invisible Republic) republished by Picador.
“He would pull these songs out of nowhere,” Robbie Robertson said. “We didn’t know if he wrote them or if he remembered them. When he sang them, you couldn’t tell.” That, in the basement tapes laboratory, is the alchemy, and in that alchemy is an undiscovered country, like the purloined letter hiding in plain sight.
Twenty-six years after they were made, years during which Bob Dylan had, it seemed, long since lost all maps to any crossroads beyond those within the ever-diminishing confines of his own career, the basement tapes were creeping up and out of their laboratory as if for the first time. Without knowing quite what I meant by “a laboratory,” I tried the notion out on Robbie Robertson, a friend since the early 1970s. “No,” he said. “A conspiracy. It was like the Watergate tapes. A lot of stuff, Bob would say, ‘We should destroy this.’ ”
(Dylan) came down to the basement with a piece of typewritten paper - and it was typed out - in line form - and he just said ‘Have you got any music for this?’ I [Richard Manuel] had a couple of musical movements that fit, that seemed to fit, so I just elaborated a little bit, because I wasn’t sure what the lyrics meant. I couldn’t run upstairs and say, ‘What’s this mean Bob? Now the heart is filled with gold as if it was a purse.’
Full excerpt here