You can see why Peter Ackroyd would pick Wilkie Collins as his next biographical subject. Everything about that odd little man makes him perfectly fitted to roam through Ackroyd's imaginative universe. For a start Collins was a Londoner, bred in the city that has always been Ackroyd's first love, a major character in so many of his novels and non-fiction books. More specifically Collins, who was born in 1824, lived at that time which Ackroyd does best, the moment when the world became recognisably modern. Collins's  and Ackroyd's  high-Victorian London is a place of hectic togetherness and soulful isolation. Bodies pack into music halls, jostle on buses and in taverns. But they are forced apart, too, obliged to scuttle home to single rooms and separate lives, reaching out to each other through the impersonal touch of the postal service and the law. It is a landscape where a man can deliberately lose himself, yet still meet his own shadow coming round the corner.

And this, of course, is exactly what happened to Collins. Famously he had not one but two partners, a fact that neatly points up the endless "doubling" that occurs in his novels. Just as the dopplegängers Laura Fairlie and Anne Catherick change places in The Woman in White, so in real life "Mrs Graves" and "Mrs Dawson" were stowed away in matching Marylebone households between which Collins shuttled like a dutiful beetle. In a more typical Victorian life story  that of Collins's mentor Dickens, say  one woman would have been the wife, all pudding-like convention, the other the true love, the secret passion. But Collins was a lifelong bachelor and chose to run his two illicit households in strict equilibrium, fiddling the census return, fibbing to landlords, tweaking the rentbooks, demonstrating all the while how easy it was in this new age of scrutiny to slip through the net. The two women knew about each other and appeared to tolerate the situation, although at one point "Mrs Graves" did flounce off to marry someone else. She was soon back, though, restored to Collins's tentacular life of aliases and sliding doors.

All this exhausting flim-flam was designed to hedge the time and space that Collins needed for his art. Critics would have sniggered to hear this, preferring instead to think of him as a "machinist", a glorified hack who provided schlocky narrative by the yard, snipped to fit a variety of formats: the three decker, the serial, even the stage. The public, though, was completely in thrall to the what-happened-nextness of Collins's addictive prose. Clerks read him feverishly when they should have been working, young girls gulped him down during long afternoons, while matrons consumed him dreamily in bed. Those who couldn't get enough of him were able to extend the delicious spell by buying Woman in White bonnets, dousing themselves in Woman in White perfume and dancing Woman in White waltzes.

His feverish work rate and complicated private life (you can't help thinking that it would have saved a lot of bother if he'd simply picked one woman and married her in the usual way) meant that Collins's health was always wretched. It is difficult, though, to know exactly what was wrong with him. Gout and syphilis are likely suspects, but there was also a deep well of neurosis that could account for the aches and pains that chased around his badly put-together body. None of this was helped by an escalating addiction to laudanum. By the time Collins was being pursued by a woman with green tusks, you sense he was in big trouble. Already walking with a stick at the age of 30, by middle age he was bent and broken, wrapped in crusty bandages that oozed with deep metaphor.

All this is cracking stuff, full of the mess of 19th-century bohemian life that the Ackroyd of 20 or 30 years ago would have tackled with relish, not to mention excellent prose. What a shame, then, that he has managed to do something else entirely here, which is to make Collins sound slightly dull. This biography appears in the "Brief Lives" series that Ackroyd writes for his longtime publishers, Chatto & Windus. The rationale for each slim volume is to produce a 200-page biographical sketch, aggregated from existing accounts (so no slogging in the archive). In some ways this makes sense. About 10 years ago publishers realised that there was a dwindling appetite for doorstep cradle-to-grave narratives with their whiskery genealogies and forensic attention to what the subject had for breakfast. What was wanted, so the smart new thinking went, was for a sprightly author to provide a personal take, a private view, a sideways glance at his subject. The value of a brief life by Ackroyd, then, would lie not so much in the story as in its telling.

But this is not what he gives us here. Instead of the "inimitable verve" that the cover blurb promises, his voice resembles that of a bored clerk, as bored as that of young Wilkie Collins who slaved away at Antrobus & Co in his early days, copying invoices and bills of lading. Ackroyd shuffles through his subject's life, mangling some dates and repeating ideas about "the Victorians" that were old-fashioned 25 years ago. You probably know the sort of thing. These "Victorians" are a uniformly stuffy crew who sit around in starched collars, probably singing hymns, waiting to be shocked by the likes of Wilkie and his pals, a raffish crew who drink too much, take their mistresses to France and write the naughty books that will outlast them all. If it sounds picky to ask that Ackroyd at least acknowledge current understandings of the Victorians as variegated, multiple and contradictory, just think how surprised we would be to read similarly lumpy generalisations about ourselves, the "Second Elizabethans".

Twenty years ago Ackroyd's biography of Dickens crackled with intellectual and creative daring. In places, to be sure, it misfired. There were some fictional passages, an awkward nod to the postmodern moment, that were sensibly dropped from subsequent editions. Still, it brilliantly suggested a Dickens for the late 20th century. Five years earlier Ackroyd had shown ingenious cheek when he wrote a life of TS Eliot without being allowed to quote from the great man's published work or private writings, demonstrating just what a whip-smart biographer could do with paraphrase and careful contextualisation. Meanwhile his bio-fictions of the 1980s  Hawksmoor, Chatterton, The Last Testament of Oscar Wilde  were born at a time when the membrane between fiction and non-fiction had become so fragile that you longed for a fat finger to poke through and see what happened when the two were allowed to mix. In all three cases Ackroyd's work mattered deeply because he understood, a beat before everyone else, that the only past we have is the one that we are able to imagine for ourselves.

So what a shame Ackroyd isn't able to bring that same bright perception to this short life of Collins. This is doubly disappointing because we are currently in thrall to the novelist's particular brand of stylish sensationalism all over again. Think of how Sarah Waters's Fingersmith reworks The Woman in White, or the fact that Mr Whicher, the real-life hero of Kate Summerscale's Samuel Johnson prize-winning book, was a model for Sgt Cuff in The Moonstone. Then there's the way that Susan Hill's endlessly reversioned The Woman in Black (itself in conversation with Collins) is having another little moment. We are under Wilkie Collins's spell right now, and we need someone to show us why.

 Kathryn Hughes's biography of Mrs Beeton is published by Harper Perennial.