'It was 20 years ago today" came and went, as anniversaries do, a long time ago.

Widows, heirs, two elderly men and a gargantuan business empire are all that truly remain. The music is repackaged periodically, as technology dictates, and the profits still flow. Memory does what it does best with those little tricks. Does more need to be said?

When the question comes in Mark Lewisohn's direction, he will have my sympathies. In a parallel world, I have been asked the same about Bob Dylan. Don't we know it all, or know as much as we need to know? Here are 946 pages in the first volume of three on a pop group whose recording career did not even outlast the decade on which they were imprinted. This is a surfeit, surely?

History was once told as the story of kings. Sometimes a politician or a war would be thrown in to give meaning to the dates. In the last century, historians began to try to tell the tale through the experiences of common folk. "Remarkable men" would no longer do. So what transpires if you chance upon four remarkable individuals who were common as muck and became an experience held in common by half the world?

Lewisohn does not need to justify himself. The fact that half a century and more has elapsed with The Beatles still pre-eminent in art and folk memory answers most questions. For some of us, their advent was akin to a suspension of nature's laws. In one moment the world was drenched in black and white, the next - as though a switch had been thrown - there was existence in living colour. Sadness, like nostalgia, comes from the fact that it could only happen once.

Lewisohn knows it all and means to tell all. Of itself, this is fascinating. Every biographer faces the research question, the inquiry over the relative worth and relevance of facts. You might have worked for a week hunting down the veracity of a famous anecdote. Are you therefore entitled to inflict the fruits of your strange labours on the reader? Is there such a thing as "definitive"? Who needs to know everything? Here Lewisohn offers a grand refutation.

In this book, there is indeed everything you could ever wish to know about four young Liverpudlians who got lucky. If it helps to listen to John Lennon with a knowledge of his strange, sweet and troubled mother Julia, this is your book. If you enjoy music better by understanding that Ringo was the real working-class hero, Lewisohn is your man. Why was George both bolshy and shy, McCartney implacable? Lewisohn has story upon story to tell.

Perhaps I'm partial. I'd suggest, though, that Tune In is a triumph. At times it reads like a Victorian novel, like something George Eliot would have attempted if she'd had the luck to find the Cavern in her lunch-break (and George was that kind of girl). At times it illuminates a Britain that was gone to dust in the moment it was coming into being. Music aside - spot on, in all respects - the book is alive to class. And it reads as though it is a brief 300 pages long.

I knew, vaguely, that Lennon was fond of Scotland because of his messed-up childhood. I didn't know that his attachment to Edinburgh ran so deep. I knew that Richard Starkey had periods of childhood illness. I didn't know that loneliness turned him into a kind of Gunter Grass drummer, bereft of education and only able to express himself behind the kit. I knew a little about the vast McCartney tribe, but little of how their enfolding, relentless ebullience shaped those songs.

There are many bad books about The Beatles. Pop sustains itself with cheap cliches and these four invented that game. But Lewisohn is a fan who refuses to write like a fan. He is dispassionate, honest, yet unafraid to let you know his research is oceanic. For one - just one - he tells poor Brian Epstein's early story with a humanity that is too often missing in the accounts of what it meant to be homosexual in Britain when the law stalked gay men.

What can't be told - again, I sympathise - is the essential secret. It goes like this: "How did that happen?" How did those four, no better than they should have been, become the kings in our modern story? Lewisohn does much to explain the peculiar pieces of luck that shaped four characters and four talents. He then describes the jigsaw that cut a Lennon to fit a McCartney. He can't tell you where art came from. Instead, he can do the essential job: who, when, where.

Lennon was, as the parlance goes, screwed up from the start. McCartney had the kind of vanity that comes with rude, honest mental health. Ringo needed - needed emotionally - to drum. And George Harrison had no other means by which to articulate his shining scepticism. All it took then was music. Nothing much.

Lewisohn is good on that. He is especially good on describing a world in which, for these young men, there was precious little else. If you grew up in Nottingham or Glasgow, you might want to tell the author that Liverpool was not the only neglected industrial city to have been bombed back into depression and defiance. But Tune In is brilliant in describing the addictive power of rock and roll when there was no imaginable alternative in a doomed town.

In this long and winding narrative the music figures almost as a street drug. The contraband was derided, "dangerous", hard to come by. Sensible boys didn't do that kind of thing. Yet each of these four said, probably in Lennon's voice: "I'm having some of that". What's more, they would not be denied. There was a time when being a Beatle involved some very stupid choices.

Strange as it sounds now, John's Auntie Mimi, rock of his early life, had hopes for him in one of the professions. The McCartneys expected no less of James Paul. George was never going to end up on the buses, like his dad; he would "better himself". Ringo, though, should have been just another lippy Liverpool docker.

The music said otherwise. Dylan, talking of his early vain efforts, called himself "an expeditionary". You get that same sense from this marvellous group portrait. It doesn't tell the story of art, but the story it tells makes Lennon's In My Life sound like the necessary soundtrack. They saw colours when the rest of us were still peering into the gloom.

The book involves one strange effect. It tells stories of a Britain long gone, yet everything it contains feels oddly present. Could music be the only difference? If that's the case, historians will have to reset their assumptions. The usual idea is that circumstances caused rock 'n' roll. Perhaps it was the other way around?

Nothing else would explain The Beatles. Mark Lewisohn's achievement lies with the fact that he never tries to "explain". He is not argumentative, like some who write about Bob Dylan, but he turns up the colours in a world that has once again faded to grey.