It seems scarcely credible that when the three surviving members of Led Zeppelin reunited for their one-off show in London in 2007, 20 million people applied for tickets. Though they bestrode the world like a priapic, all-devouring monster of depravity in the 1970s, the years following their split in 1980  caused by the death of drummer John Bonham  were not kind to them. Critical orthodoxy held them to be dinosaurs, representatives of all that was worst about rock excess in both behaviour and music.

Offstage, one horrific tale holds that some combination of members of the band and their road crew whiled away an evening in a Seattle hotel inserting fish of disputed species into a woman's vagina. Onstage, Nick Hornby recalls in his book 31 Songs a guitar solo so long he was able to leave Earl's Court arena, pop to the pub for a pint and a game of pool and return before the end of it. The band's reputation was further diminished by a disastrous, under-rehearsed appearance at Live Aid in 1985, which they did not allow to be included on the 2004 DVD release of the concerts.

Barney Hoskyns's exhaustive history doesn't skimp on the horror  and by God, there's enough of it  but nor does it turn Led Zeppelin into a cartoon of decadence. It never forgets that behind the caricature was a wonderful band, lithe and limber, never the lumpen metal pioneers that they are sometimes branded. Where most bands can be reduced to guitarist, singer and supporting cast, all of Zeppelin's four members  as Hoskyns's 128 interviewees, including the band and intimates, as well as close observers, make clear  were crucial in making them extraordinary.

The heart of Zeppelin's story is the relationship between Jimmy Page, the guitarist who formed the band in 1968, and their manager, Peter Grant. Grant was desperately protective of his band, and succeeded in altering the balance of the music industry in favour of artists, at least those powerful enough to wrest control from the promoters and labels who had previously made the lion's share of the money. In the process of doing so, though, he became powerful himself, and was corrupted absolutely in the process.

Page authorised and trusted Grant to do anything to protect the group's interests, and as the pair got more and more heavily involved in drugs in the late 1970s, so the deeds so casually committed in Zeppelin's name  threats of violence, actual violence, near-blackmail of business associates  became more and more appalling. "There's a one-word answer: cocaine," says one of the band's press liaison people of the air of menace that surrounded them by 1977. "Or another related word: paranoia. Peter and Jimmy were the most paranoid people I have ever met in my life, bar none."

Rock mythology holds that Page was somehow a "dark" force: he collected occult books and artefacts, bought Aleister Crowley's old mansion and opened an occult shop. But it seems less likely that Page's behaviour was caused by some sort of deal with Satan than him apparently being a fairly nasty piece of work, selfish and mean and manipulative.

When your reputation is being defended by a former teenage groupie on the grounds that you did not actually laugh while she was being assaulted by another teenage groupie, you've probably lost all contact with normal levels of acceptable behaviour. Not that he was alone: one of Hoskyns's chapters opens with the instructions to journalists covering their 1977 US tour: "1 Never talk to anyone in the band unless they first talk to you. 1A Do not make any sort of eye contact with John Bonham. This is for your own safety."

If there's a hero in Trampled Under Foot, it's Zeppelin's singer, Robert Plant, portrayed as the light to Page's darkness. His discomfort with what the band became is evident, intensified after the death of his five-year-old son in July 1977. But still, across page after page, one can't help but think: he was there, why didn't he do something? It's a point made by someone present at one of the most notorious incidents in the band's history, when a security guard was twice badly beaten up backstage at a gig in Oakland: "Plant seemed like the only decent human being there, although there were no innocents. There were no innocents."

So why did those 20 million people apply for tickets in 2007? The answer lies in the records to which this book sent me scurrying back: in the sinuous, thrilling interplay of instruments in "Nobody's Fault But Mine"; the preposterous bombast of "Kashmir"; the magnificent idiocy of "The Immigrant Song" ("The hammer of the gods will drive our ships to new lands!"); the brutal combination of 60s freakbeat and proto-metal in "Communication Breakdown". It's the songs that made Led Zeppelin the biggest band in the world; and it was being the biggest band in the world that made them a horror show.