There is no "fiction" and "non-fiction". There is only the written and the unwritten, and anything that is written is in some sense fiction. So, no real need to waste time trying to decide what kind of book this is. Waterstone's and Borders should stock it in as many sections as they can - reportage, biography, history, sociology, among the novels - and pile it high inside the door. Not just because of the quiet urgency of its message, but because Foreigners is immaculately written, with a subtlety familiar from Caryl Phillips's earlier work, but here almost superhuman.

The three lives in question are not entirely English at all, and it is their Englishness that stands in question. The first is Francis Barber, a black slave casually manumitted to Dr Johnson who subsequently became the great lexicographer's companion and protector. It is a relatively familiar story, but told here with virtuosic control as the unnamed narrator moves back and forth in time between Johnson's hasty, unadorned funeral, the troubled sequence of the doctor's final years, and his own visit to Johnson's native Lichfield to meet Frank's widow, who lives and teaches there.

It's no accident that Phillips's first section should be so concerned with the making and codifying of the English language, for in the narrator's own choices of words - "negro", "sooty", "ebony", "mongrel", "piccaninny" and so on - is the armature of a national attitude to race that echoes down through the next two centuries and into the urbane and classically educated voice of Enoch Powell.

The second life is that of the "mongrel" Randolph Adolphus Turpin, the mixed-race fighter who became Britain's first black world boxing champion when in 1951 he beat the elegant Sugar Ray Robinson, but committed suicide 15 years later, debt-ridden and aged only 38. The press had flocked to Randy Turpin during the years of his success; he had friends as long as he had money. By the time he was obliged to eke out an undignified living in the wrestling ring, they were all gone.

The voice here is the closest to neutral reportage, a dramatic shift from the proud, hard but not irredeemable narration of the opening section. Here the "I" is Phillips himself, speaking to the family almost as if he were writing an article for a magazine, writing in short sentences, painting pictures but not over-dramatising events.

The third is David Oluwale, who came from Nigeria as a stowaway in 1949 and died in police hands two decades later in a case that continues to stand as a paradigm of the institutionalised racism of British society. Here the prose is an extended apostrophe to a man betrayed by an empty promise and then failed by the "system", a man who lies buried in a crowded Catholic (why?) cemetery at the top of a hill in Leeds. An ironic summit to a life, and a twisted Calvary.

Scholars like David Dabydeen and others have done sterling work by throwing light on the not so much hidden as overlooked history of black men and women in Britain, those dark faces who look out of genre paintings, historical scenes and the fringes of portrait, but who seem to exist there without biography and without context, merely as part of the chiaroscuro. Phillips, though, has a novelist's instincts and an eye for the detail and for the language that will softly reverse the monochromes not into a negative but into something like a solarised image where the edges of words and of objects are picked out with a strange glow.

There is an irony that resonates through the book and changes the moral cast of the two later sections. It turns out that Betsy Barber is not a widow after all. Her husband has not thrived, it is true, but is not yet dead. He does, however, lie dying in the Stafford Workhouse Infirmary, his monetary legacy from Dr Johnson squandered, his gift of "liberty" meaningless. The narrator concludes that "all ebony personages" should be persuaded to return to Africa or Jamaica and is confirmed in his intention to investinthegloriously named Province of Freedom, a version of Liberia where every sooty mongrel might find a more amenable life and resting place.

His last act is to stay on at Burntwood, near Lichfield, to give Betsy Dr Johnson's watch, in order that she might pawn it for some much-needed cash. It is by no meansaheavy-handed symbol, but how it clangs with new meaning when one goes back through these short texts: for once add time to language, and money to the pair of them, and there you have the vectors of a whole 250 years of history.

Phillipshasnever daredtowritemore simply or with higher risk ofmisunderstanding. Foreigners is a book that must be, and can only be, re-read.