London: Simon & Schuster: 1998; £16.99 hc; 393 pages

On 19th August 1987 Michael Ryan carried out a massacre in the normally quiet streets of a little Berkshire town called Hungerford. Christopher Priest happened to be in Hungerford that day; though never in any danger, the proximity to violence clearly stayed with him. Eventually it began to emerge as a novel to be called The Cull, but as he began writing there was another massacre. On 13th March 1996 Thomas Hamilton took a collection of guns into a school in Dunblane, Scotland, and killed 16 children and their teacher. Thus part of the genesis of The Extremes, as The Cull eventually became known, a novel that is darker and more violent than anything else Priest has written.

The rest can be found in Priest's other novels, particularly the string of haunting, uneasy psychological dramas from A Dream of Wessex (The Perfect Lover, 1977) up to The Prestige (1996). Themes and images that recur throughout those books all find an echo in The Extremes. In the opening scene, for instance, we see a young Teresa Gravatt, child of a mixed American-British marriage, growing up on an American army base outside Liverpool. She is lonely, her only friend an imaginary playmate, until Teresa finds her father's revolver and even that comfort is shattered. It is a curious moment, detached from the rest of the novel, seeming to cast a clear but not necessarily significant light on the psychology of the central character; until Priest returns to it briefly right at the end of the book, refracts the scene through a different perspective, and suddenly we are plunged into the vertiginous world of doubles that echoed and re-echoed throughout The Prestige.

Teresa has returned to the States, grown up, become an FBI agent and married fellow agent Andy Simons. Then her husband is killed when a gunman goes wild in a small town shopping mall in Texas. By a curious coincidence there is a similar shooting incident in Britain on the same day, and Teresa, on indefinite leave, returns to Britain as if she might find some solution for her own grief in the small Sussex town of Bulverton. Of course, in Priest's work there is no such thing as coincidence: each twinning of people or places or events is no more than a tightening of the noose around our notions of reality. The echoes between Bulverton and Texas prove to be greater than anticipated, but the network of interconnections can only be completed by stepping outside consensus reality. And here we cross the second boundary between parallel worlds in The Extremes.

As part of her FBI training, Teresa used Extreme Experience - ExEx - a form of virtual reality in which she became a participant in real crimes of the past, learning through violent experience how and when to react. Now ExEx is available to the public - there is an outlet on the edge of Bulverton - and even more than pornography the most popular scenarios involve violent crime. In fact, a blandly arrogant team from GunHo, the world's largest ExEx corporation, is in Bulverton signing up the survivors of the massacre so their memories can be used in creating a new scenario, and resenting the way that Teresa's researches are muddying the water. But Teresa discovers that ExEx is already implicated in the crime. The gunman, Gerry Grove, first killed a family picnicking in the woods, then he went to ExEx and there was a gap of several hours not fully accounted for in any of the reports before he resumed his shooting spree. More curious still: he had two guns, he used those two guns to commit all his murders, the same two guns were with him when he was finally killed; yet two identical guns were later found in the boot of his car, abandoned outside the ExEx building. It is this inexplicable doubling that gives the first clue as to how strange this story will become.

For Teresa doesn't just investigate ExEx, she enters their scenarios, learns how to change the behaviour of the characters she inhabits, explores the limits of their digital worlds, eventually she is able to converse with the character in a porn scenario. Gradually, as the novel progresses, Teresa is able to link together scenarios until they build into a true parallel reality, one with a chilling ability to have a retrospective effect on our own everyday world. Then she enters the Bulverton massacre scenario, and finds herself inside the mind of Gerry Grove.

One of the figures that is repeated time and again throughout Christopher Priest's work is the psycho-sexual triangle: one woman finds herself caught between two men who, in coarse terms, represent good and evil. It is there in A Dream of Wessex, for instance, and again in The Glamour (1984), and a distorted version of it is woven into the pattern that forms the final third of The Extremes. Here however, for the first time, the sole viewpoint character is the woman, Teresa (it helps that in this instance both men, her husband Andy Simons and the murderer Gerry Grove, have been killed before the novel opens). The nasty, brutish interplay between Teresa and Grove is probably the best part of this very powerful and effective book.

Priest has always avoided violence. Though there is usually drama enough in his work, outright brutality plays next to no part in any of his novels before this, yet here it is the very warp and weft of the story. He handles the change of subject well; there is, for instance, none of the queasiness with which he approached perverse sex in The Quiet Woman (1990) and which irreparably damaged that novel. Rather he maintains his usual cool, distanced approach. There is no overt emotion in his language, which suits the story extraordinarily well; the most extreme human behaviour and the most outrageous twist in the fabric of reality are both made to seem believable by the simple factual language with which they are described.

There are times, still, when the simple thriller that The Cull might have been peeks through the dense layering of The Extremes, but when it does that only emphasises the complexity of this novel. In subject matter, The Extremes clearly suggests that Priest is moving, belatedly, into the realm of the cyberpunks, but he is in fact approaching virtual reality with a very different aesthetic. Where, for William Gibson or Pat Cadigan, for instance, the computer creates an other world clearly different from our own in its colours, shapes and most commonly its spaces; for Priest, virtual reality is indistinguishable from our own reality. One enters clearly into a here and now that may have its boundaries, but which within its narrow compass is a mirror for our world. Another example of the doubling that echoes throughout his recent work, what is important about this version of virtual reality is that the borders between the realities become increasingly hard to see. There is no other world, this is not a place that we enter as we might enter Gibson's cyberspace or Greg Egan's Permutation City; Priest does not allow that escape, only endlessly different perspectives on the same scene. Virtual reality has been a science fiction toy for years, brought out whenever a writer wants to play with reality. Here, perhaps for the first time, it becomes a precise surgical instrument slicing through our slender grip on who and what and where we are.

As he has done triumphantly in The Affirmation and The Prestige, Priest quietly overturns everything we might rely on when we look for the surety of our own identity, the solidity of our world. Whether this makes the book science fiction is, of course, another matter. In the early 1980s Priest famously left science fiction, despite going on to write such haunting books as The Affirmation and The Glamour, which teased our notions of reality. He has gone on record as not wanting The Extremes published as science fiction, more, I think, because he is afraid of the narrow expectations such categorisation brings rather than from any dislike of the genre. But after winning the World Fantasy Award as well as the mainstream James Tait Black Memorial Award for his previous novel, The Prestige, and with the powerful and at times harrowing take on virtual reality that he achieves with The Extremes, it can not be denied that Priest's work is once again central to any appreciation of what the genre is doing today.