Picture a writer of British extraction, born and brought up in an exotic foreign country, largely educated in England, hailed as a literary comer with the success of his first book, published when he's in his 20s.

If you're thinking William Boyd, born in Ghana, sent to boarding school in Scotland, possessor of a postgraduate degree from Oxford and the author of A Good Man in Africa, which earned the Whitbread in 1981, you're right.

But then there's Logan Mountstuart, born in Uruguay, educated at an English boarding school and Oxford, and only 22 years old when his first book, a biography of Shelley, is published to critical acclaim.

The difference between them? First, Logan is the fictional protagonist of Boyd's sweeping, rumbustious, and surprisingly tender new novel, Any Human Heart (Knopf). Second is their birth dates: Boyd was born in 1952, his hero in 1906. Boyd's novel chronicles Logan's life as it spans the 20th century, during which World War II intrudes with harrowing consequences, leaving Logan bereaved and unable to produce another book that equals the brilliance of his initial effort.

Boyd has been a luckier man. He's enjoyed a prize-studded literary career with stints screenwriting in Hollywood and teaching at Oxford, has been an expatriate in dozens of countries and has maintained a devoted interest in the world of contemporary art (his first ambition was to be a painter, and he's currently on the board of the magazine Modern Painters). In England, he's ranked in the company of Amis, Unsworth and McEwan. Reached by phone at his home in London on an evening when snow is falling ("a rare event"), Boyd is ensconced in his study, which he describes as crowded and untidy, with books piled everywhere. "It's like an installation. I'm going to be swallowed up by books in a year or two," he says, with no apparent regret.

He's surprised himself about the hold that Logan Mountstuart has had on his imagination. "I've always said that I would never write a book about another writer. But being a novelist, I'm fascinated by the breed." He names the preeminent figures of Evelyn Waugh, D.H. Lawrence and others of their ilk, but says that there was a second division, made up of such writers as Henry Green, Lawrence Durrell and William Gerhardie, who are even more interesting to him. Gerhardie is a new name to us, and perhaps to most American readers. Boyd explains: "He was the hottest young talent of the 1920s. He influenced Waugh and Green. In my opinion, he's the case study of what can go wrong." Gerhardie, it turns out, published 10 novels; the first few were literary sensations, but his later work failed. His last novel appeared in 1940, and he died in 1977. "He had 37 years of silence," Boyd says.

Logan Mountstuart is not Gerhardie, Boyd says, but Waugh and Green and Gerhardie are blended into his portrait. All had similar lives: boarding school, Oxford, "the transforming '20s," World War I. The first time Logan appeared in Boyd's work was in the short story, "Hotel des Voyageurs," in the collection The Destiny of Nathalie X. "I began to imagine him more and more, and he pops up again in my little spoof memoir, Nat Tate [1998]. He invaded me, and eventually I realized that in the life that Logan might have led, I had a kind of exemplary figure."

In tracing Logan's life through his journals, chronologically from 1923 to 1991, Boyd creates a character of extraordinary appeal. High principled, brave, naïve, stoic, compassionate and resilient, Logan is swept up in the historical, social and artistic crises of a turbulent century. Logan brushes up against many of the era's leading writers (from Virginia Woolf to Joyce, Hemingway to Ian Fleming) and artists (most crucially Picasso); takes part in the Spanish civil war; is absorbed into the circle of the Duke and Duchess of Windsor; makes a bad marriage and then a good one; becomes a spy during WWII, and is captured and imprisoned in Switzerland; turns to journalism to earn a living in London after the war; runs an abstract expressionist art gallery in Manhattan; teaches English lit in Nigeria during the Biafran conflict; back in London, is reduced to eating dog food; falls into the clutches of a German terrorist gang, and finally finds peace in a tiny house in France.

However hectic his life journey may sound, Logan is more than a cartoon Zelig or an outsized Wouk-like hero who turns up at all the critical junctures of history. As his journals reveal, he's a fallible man who makes mistakes in his career and affairs of the heart, but he manages to retain his zest for life. He also fumbles into some dangerous errors of judgment inspired by his burning sense of justice (often ironically misplaced) and his desire to make a difference in the world.

Boyd clearly admires his character's moral commitment and stresses his universality. "Noble impulses are a very good thing, but their nobility doesn't guarantee success," he says. "We don't despair, however; we don't repine; we continue to try to do the right thing. But we're often thwarted by bleakly comic events that are part of the human condition."

Actually, Boyd has ventured down this road before. The life of the filmmaker-hero of The New Confessions (1988) also spans nearly a century, during which the chaos of war plays a pivotal part. And the eponymous Nat Tate is an American artist whose life intersects with the giants of the New York art world. Boyd says that he's just recently come to understand that the three books form a kind of pattern, a triptych, as it were. "One is written as autobiography, one is memoir and third is an intimate journal. Now I realize that the journal is the literary form that's closest to the way in which we all actually live. Other ways of writing about a life are too shaped, neat and manipulated, and we all know that life is not like that. Life is random, and in a journal you get a sense of life being lived on the run, day by day." In their candor and innocence, Logan's journals reflect his on-the-spot responses (he thinks Hemingway's A Farewell to Arms is "an embarrassing disaster," for example), and the chance meetings that, in retrospect, turn out to be the pivotal events in his life.

The unpredictable accidents of life and the long-term consequences of small events, is, of course, a sustained theme in all of Boyd's fiction. "You realize yourself as you look back on your life that the events are totally chance driven. The job you got, the person you met. The view ahead is a void, and it's all to do with happenstance and good luck and bad luck."

In Logan's case, the major factor is the war, which, he feels, irrevocably maimed him and his generation. On the other hand, Logan also exemplifies another of Boyd's recurrent themes, that happiness is transitory, yet the memory of happiness counts for everything. At the end of his picaresque life, Logan reminisces about the short span of his domestic life with his wife and baby daughter. "Those were the years when I was truly happy. Knowing that is both a blessing and a curse." It's a moving passage, and it's what makes Logan so human and his story so profound.

The book's title is from an epigraph by Henry James: Never say you know the last word about any human heart. Boyd confesses that titles give him terrible problems. "I always feel that the right title is a kind of blessing on a book. I could never let a book go until I was happy with the title. Sometimes I get them early on; other times it's panic station." Any Human Heart came right at the end. "When you get it, you know it's right. I was so pleased."

Boyd never sells his books before he's finished them. At 495 hefty, liberally footnoted pages, Any Human Heart took only 18 months to write, but he'd been thinking about it for about three years. The germination period of all his novels is long, he says. "I find that if I spend maybe two years thinking about the novel I can sit down and write it in confidence."

Writing this novel also gave Boyd the chance to have a go at his bête noirs. Virginia Woolf comes across as a strident and venomous bigot. Boyd takes pains to stress that he's quoted her exact words, which she penned in her journal, about the wife of novelist and critic Cyril Connolly. He seems gleeful at having taken on the current icon of feminists and movie makers, but he's even more delighted at having skewered the Duke and Duchess of Windsor, whom he describes as "one of the most unpleasant couples ever." For those who don't remember the murder of Bahamian millionaire Harry Oakes in 1943, during the duke's tenure as governor of the island, reading about the crime from Logan's (and Boyd's) point of view will be a shock. Boyd says he did a lot of research about the case, and feels he definitively proves that the duke conspired to derail the cause of justice. "I'm interested in demythologizing him. He would have seen an innocent man hanged if that had protected him," he says, with some heat.

Closer to home is Boyd's satirical treatment of the inequities of the writer's lot. Logan's friend of boarding school and Oxford days, Peter Scabius, becomes successful, rich, celebrated (and knighted) as the result of the thrillers he turns out with facile frequency. Boyd says that Scabius, in his shameless manipulation of public attention, is "sort of a two-dimensional portrait of a vulgar Graham Greene. It's a phenomenon we all know. We think, can't the rest of the world see through his fraudulence?" A weary but not unpleasant laugh rumbles along the phone line.

The character of Scabius invites ridicule, but the antic black comedy that crackles through some of Boyd's previous books is subdued here, and while ironic humor is not lacking, this novel presents a more compassionate portrait than Boyd has given us in the past. Boyd's mature vision reconciles good luck and bad into a redemptive vision. "Logan does achieve a kind of serenity at the end of his rackety life. There's no sense in his mind that he hasn't lived well. Even though it seems that success came early and it's been a long slide downhill, he's actually lived a good life," Boyd sums up.

There's been no slide downhill for Boyd; in fact, this novel finds him at the top of his form and poised to produce distinguished fiction for some time to come.