The truth is beneath us in London Under

London Under

What we have here is Peter Ackroyd and subterranean London. One is deep, dark and mysterious. And the other is subterranean London.

Ackroyd's fans may feel that the magus of the metropolis was born to write about what lies beneath the city streets. After all, in his prolific fiction and cultural history alike, he has goosed his readers' flesh with intimations of what goes on in the unfathomable depths of the Big Smoke. Sure enough, 11 years after he produced London: The Biography, he now examines the hidden organs of the capital, its "nerves" and guts and bowels. This is London: The Biopsy.

If future archaeologists dig this book up from the pressing London clay, they will find a gem of tightly compacted scholarship. Whereas London: The Biography was the size of a London phonebook, this is almost as slender as an iPhone.

Admirers will be delighted to find the master twirling his moustaches and swirling his cape in much-loved fashion. "The sewers are places of universal defilement," he writes, "this was the heart of darkness." Ooh-er!

Tube tunnels where the hair and sloughed-off skin of commuters accumulates; silver vaults; secret bunkers - Ackroyd gets down and dirty with them all. Except that he doesn't really do down and dirty. The writer who famously paced the streets like his hero Dickens is putting his feet up these days. "I wish to descend and explore the lower depths," he says but, as he has made clear to interviewers, he does this without leaving his desk.

It's impossible to imagine that Peter Ackroyd, of all people, is tired of London, but there are suggestions of fatigue nonetheless. "It would be weary work to enumerate all the buried wells of London," he sighs. By the end of this account, the reader is left in little doubt that the city below ground doesn't really do it for him. Ackroyd says that each Underground line has its own personality - the Circle Line is "adventurous and breezy" - but he doesn't use the network any more himself, and he's glad of it. "Now that I am beyond its reach, I feel free."

Of course, few of us love the Tube, and as Ackroyd proves, it's not essential to frequent London's underworld in order to write well about it. But it's a bit of a turn-up to find our foremost diviner of this space telling us in so many words: don't go there, girlfriend! We owe Ackroyd a great debt, all the same. He has memorialised London so well, it's time London returned the compliment.

Mindful of the age of austerity, let's gather up that loose hair and stray skin from the Underground tunnels, beg some jewels and precious metals from the vaults, and furnish a bust of the author for the fourth plinth of Trafalgar Square.

Stephen Smith is the author of Underground London (Abacus)