Visionary of the modern Babylon

Peter Ackroyd: With his latest book, one of our most scholarly, hard-drinking and mystical writers has crossed new boundaries

Peter Ackroyd is satanically prolific, and his output seems to be accelerating. Simon Callow performs nightly in Ackroyd's play The Mystery of Charles Dickens at the Comedy Theatre in the West End. There are the hefty biographies - of William Blake, Thomas More, TS Eliot and Charles Dickens; there's even a volume of poetry ( The Diversions of Purley). The most indefatigable fan is hard pushed to keep up with Ackroyd the novel-writing machine. So, for the record, what are the books about? The Great Fire of London? A TV director tries to film a version of Little Dorrit. The House of Doctor Dee? Researcher inherits spooky Clerkenwell mansion once inhabited by Elizabethan magus. First Light? Er, a clash between astronomers and archaeologists at a ... Milton in America? John Milton emigrates to ... English Music? The Plato Papers?

Peter Ackroyd is satanically prolific, and his output seems to be accelerating. Simon Callow performs nightly in Ackroyd's play The Mystery of Charles Dickens at the Comedy Theatre in the West End. There are the hefty biographies - of William Blake, Thomas More, TS Eliot and Charles Dickens; there's even a volume of poetry ( The Diversions of Purley). The most indefatigable fan is hard pushed to keep up with Ackroyd the novel-writing machine. So, for the record, what are the books about? The Great Fire of London? A TV director tries to film a version of Little Dorrit. The House of Doctor Dee? Researcher inherits spooky Clerkenwell mansion once inhabited by Elizabethan magus. First Light? Er, a clash between astronomers and archaeologists at a ... Milton in America? John Milton emigrates to ... English Music? The Plato Papers?

There's also the regular book review slot at the Times: clearly a dream contributor, Ackroyd never balks at bulk, tackling with aplomb the sort of adipose mega-biographies he has done so much to promote with his own works. It is tempting to believe that Ackroyd retreats at will into a series of Groundhog Days to write, turning in an effortless 900 pages of scholarly distillation in the time taken by lesser mortals to draft a shopping list.

Yet he's no hack, but a visionary. All his books flow into one another and, as he is fond of pointing out, are all part of the same project. He doesn't have a rest after writing one book: he's on to the next. Interviewed in March 1998 for his last major work, The Life of Thomas More, he was already keen to talk about London: The Biography, which has just been published by Chatto: "My next book is a biography of London, written as if it is a person. It's neither fiction nor non-fiction. It's somewhere in between."

That "somewhere in between" is what upsets the more scholarly reviewer. "An efficient cobbler," remarked Hugh Trevor-Roper of his More biography, though another reviewer got over his initial reaction ("poor More") to admit that Ackroyd possessed "a humanity so often lacking in the work of professional historians". But his Blake disappointed, despite brilliant sequences bringing to life late-18th-century London with all its smells and noise - mainly because Blake is impossible to ventriloquise.

The man who once observed: "Fiction requires truth-telling, whereas in a biography one can make things up", has not applied similar licence to his own biography, generally served penny-plain. The uninitiated, on glimpsing the blimpish, Colonel Mustard figure rolling around the more select literary parties, might be surprised to learn that Peter Ackroyd was born in Acton in west London in 1949 and brought up, by his mother and grandmother, in the absence of a father, on a council estate near Wormwood Scrubs. Young Ackroyd, the scholarship boy, attended St Benedict's School in Ealing, a Catholic public school and alma mater of Chris Patten and Julian Clary, and an establishment run by Benedictine monks who adhere to the custom of silent meals accompanied by readings from the lectern. The Life of Thomas More was a much-admired recent choice.

Ackroyd the grafter gained a double first at Cambridge and won a scholarship to Yale. Back in London at the age of 23, he fell into the job of literary editor at the Spectator after replying "Drink" to the trick interview question "Do you have any problems?". He remained there until the publication of his first novel, The Great Fire of London, in 1982.

A biography of TS Eliot betrayed no hint of his subsequent peculiarities in the genre; it was in Hawksmoor(1985) that the triffid began to put out its glamorous, unhealthy blooms, as the author waded out into what he has called "the viscosity and strangeness of the past". With Hawksmoor, Ackroyd not only brought the then completely unknown Iain Sinclair to public notice with a generous acknowledgement, but arguably inaugurated the autopsy school of crime fiction. We now take morgue porn for granted, but it seemed repellent then, to judge from the reaction of James Fenton, reviewing the novel for the Times: "a ... forensic scientist ... cuts open the body and places a thermometer in the liver to establish the time of death. That detail seems to have been chosen with a certain glee ... now I'm going to be sick."

In 1990 Ackroyd was widely felt to have overreached himself with a 1,000-page biography of his beloved Dickens which featured fictional dialogues between Chatterton, Wilde, TS Eliot and his subject. For all its imposing bulk, it was eclipsed by Claire Tomalin's much slimmer, more acute The Invisible Woman, a stunning reconstruction of Dickens's affair with the young actress Ellen Ternan, the implications of which passed Ackroyd by. "The thing we don't get about [Dickens] at all," confessed Simon Callow recently, "is his sexuality"; to which Ackroyd replied: "It's not in my book ... I tend not to speculate about sexual matters." Still, the venerable Anthony Burgess bought the Ackroyd line: "All the evidence is against Dickens's copulating with Ellen Ternan," he announced in a glowing review.

Booked up like an opera singer for years in advance, Ackroyd has recently shown hints of fatigue with the schedule that netted him a reported £1.25m for eight books, biographies of Defoe, Shakespeare and Turner among them. "I am so intent on pursuing a vision that I don't want to be bothered," he said at the time of this onerous, if lucrative commitment. "I know exactly what the books will be, and I just want the freedom to write them." But now Defoe and Turner are apparently on hold, and Ackroyd has spoken with enthusiasm about a much more shadowy and nebulous project, a book on "the origins of the English imagination", identifying "a buried Catholic sensibility or culture". Though he lapsed in his teens, he sees his spiritual, mystical outlook as a corrective to the stubbornly earthbound fictions of his peers.

Perhaps his new restlessness is linked with deep-seated disturbances in his personal life. A long-term lover, the dedicatee of his first novel, died of Aids in 1994, and another relationship ended this year, suddenly impelling him to up sticks from his house in Islington to a flat in Kensington. Ackroyd has for years been a legendary drinker. A brave male journalist totted up their quota as four whisky-sodas, four bottles of wine over lunch and brandies after. Accounts of his flirtatious behaviour make Ackroyd sound like Uncle Monty.

Last week, he was a guest, with the other presiding genius of psychogeography, Iain Sinclair, and Claire Tomalin, on Melvyn Bragg's Front Row programme on Radio 4. Bragg listened with incredulity as Sinclair and Ackroyd explained the perfectly (to them) self-evident idea that London itself - the city - caused the heart attack which afflicted its biographer as soon as he finished his labours. "I'm a novelist, too," Bragg protested, as Sinclair and Ackroyd expounded their imaginative, poetic notions - that certain districts have their own personalities, that human beings can "catch" suicidal or radical tendencies from their surroundings; can, all unknowingly, behave exactly as their predecessors did in the same location.

"Do you really believe areas of London are ... organisms?" Bragg asked plaintively. "If London teaches us anything, it is that there is no such thing as coincidence," replied Ackroyd, grandly. Or contradiction, he might have added. When faced with rival etymologies - Pen meaning "head" or "hill" and ton meaning "spring", set against the idea that Pentonville is named after the 18th-century Henry Penton - Ackroyd surmised, "Is it possible that both explanations ... are true simultaneously?"

Peter Ackroyd has more reason than most to echo Whitman's famous formulation: "Do I contradict myself? Very well then I contradict myself. I am large, I contain multitudes." If London: The Biography is really a biography, it is the life of Ackroyd himself. He has finally metamorphosed into brick and stone.