A couple of years after graduating from the University of Maine, where he listened to a lot of classic rock and roll, Joe Hagan landed a dream gig: an internship in New York at “Rolling Stone” magazine. “I was just a young, wet behind the ears kid who showed up in the city with big eyes and didn’t know what was going on,” he says. The setting may have been glamorous but the work was pedestrian, the stuff that gets dumped on interns. Among Hagan’s tasks: cranking out copies on the office printer.

Two decades later, after various jobs in journalism including a stint covering media for the “The Wall Street Journal,” Hagan got an offer he couldn’t quite believe. Jann Wenner, the founder and editor of “Rolling Stone,” asked him to write his biography. Wenner was a formidable figure, an energetic and enthusiastic player at the intersection of music and journalism and popular culture since the magazine took off in the late 1960s. “I was in awe,” Hagan says. “My jaw was hanging down. I was in shock.”

He accepted—but not without reservations. “Wenner was sort of an irascible, unpredictable character,” Hagan says. “ ‘Mercurial’ is what people said about him.”

The book, “Sticky Fingers: The Life and Times of Jann Wenner,” came out last fall to strong reviews. “It’s a joy to read and feels built to last,” proclaimed “The New York Times.”

Its subject, in contrast, hated the book, denouncing it as “deeply flawed and tawdry.” That reaction did not surprise Hagan. “I knew who he was and what his personality was like,” Hagan says, “and he had the kind of personality that didn’t really deal with criticism too well.”

Hagan is unruffled by Wenner’s anger because, he says, he wrote a true story. “One day he’ll come around to realize that, I think,” Hagan says. “And it’s better for his legacy than a whitewash.”