Eminem soared from drug-filled poverty to adulation and notoriety

When I began writing the first serious biography of Eminem 10 years ago, he was at the height of his powers and notoriety. "Stan", his astonishing shaggy-dog tale of letters between himself and an unhinged fan – somehow made more funny and menacing by the sampled Dido's wan desire for a cup of tea on a drizzly English day – had just been released. Everybody I knew wanted to talk about it, and its singer, and once they started they couldn't stop. This was a rapper who in other songs had decided to kill his estranged Mum, absent Dad and long-suffering wife Kim, claimed to have murdered OJ Simpson and armed the Columbine killers, and, who when attacked for homophobia, rapped: "Hate fags? – yes." He was picketed by feminist and gay groups, baited by homophobic tabloids and liberal broadsheets. Eminem for one moment gave pop back its centrifugal force at culture's core. For the first and, to date, last time, I was offered sex for my ticket when he flew into Manchester to find he'd become a folk devil.

I was soon walking Detroit in his footsteps. The city looked as if it had been dredged from underwater. You could hear the wind whistling through huge roofless factories, and walk for 15 minutes in the city's gutted, once grand, heart without seeing a soul. Pacing down 8 Mile Road, the borderland between black urban Detroit and its white suburbs which he made notorious in songs and an Oscar-winning film, I saw the broken landscape that helped make him great. As Elvis did in Memphis, Eminem – then Marshall Mathers, of course – had experienced life as a minority on the black side of town. And he knew from black friends the bigger American picture. His shattered city made him understand class too; he knew which he was, and rapped for the underdog. The misogyny, beginning with his awful relationship with mother Debbie Mathers-Briggs, and continuing with childhood sweetheart and lyrical punchbag Kim, was more troubling; the homophobia too. But female and gay friends found his articulate rage universal. Sometimes they felt just like him.

He tended to attack rather than retreat when criticised, anyway, as Presidents from Clinton to George W Bush (who called him "the worst thing to happen to American youth since polio") both found. Retaliating to such slurs made him the most brave, articulate anti-government pop star during the paranoid, post-September-11 clampdown. In the year of Bush's 2004 re-election, "Mosh" lyrically decimated the new McCarthyism. In the ultimate battle rap, he called out the President: "Maybe we can reach al-Qa'ida with my speech/ Let the President answer our high anarchy/ Strap him with an AK-47/ Let him fight his own war/ Let him impress Daddy that way... no more blood for oil". Anyone who was in America at that time knows the risks he was taking, which no other pop star did with such directness. Those who've rhetorically wondered where the 1960s- and 1980s-style songs of protest and resistance are now: they were here.

Eminem at 38 has changed, as the isolation of his American fame has deepened. He writes about himself much more than his country now. Maybe he isn't like Ginsberg. But his commitment to renewing his astonishing talent, and the intense dramas of his life, are as bottomless as I first hoped. I'll be going back to my book.