Norman Sherry: After 27 years, Greene's shadow reaches the end of his life

Norman Sherry is exhausted. Monumentally, cosmically exhausted. It has been 27 years since he began working on his authorised biography of Graham Greene, and it is only now - with his subject long dead and half of his own life swallowed up in the effort - that he has put the finishing touches to the last of the three hefty volumes.

Norman Sherry is exhausted. Monumentally, cosmically exhausted. It has been 27 years since he began working on his authorised biography of Graham Greene, and it is only now - with his subject long dead and half of his own life swallowed up in the effort - that he has put the finishing touches to the last of the three hefty volumes.

He should be elated, not least because of the extraordinary praise lavished on volumes one and two. Instead, he finds himself overwhelmed by the scope of what he has accomplished, haunted by his own mortality - something he says he did not take seriously until Greene forced him to confront it - and racked by the decisions he made in his own life to bring his vast project even to this much-delayed fruition.

A "monster" is how Sherry described his work the first time we spoke, a year ago, when he was still wrestling with volume three and trying to bring it to below 1,000 pages. Not that he isn't proud of what he's done; in fact, he believes the third volume is the best thing he's ever written. For a long time, though, he doubted he would ever finish it (a doubt widely shared, as deadline after deadline passed). Even now, with the book well into its initial print run, he worries something will still happen to deny him the pleasure of next month's publication.

"Greene himself once told me he wondered if I'd live to see it completed," Sherry told me when we met, finally, in San Antonio, Texas, where he teaches at Trinity University. "For a long time I was obsessed with that idea. I lost sleep night after night. With my recent health problems, I still worry."

In the past year, Sherry, 69, has had extensive dental surgery that has left him without his top row of teeth. (At his favourite French restaurant, near his office, they have devised an entire menu of soft food to accompany his nightly bottle of Macon Villages.) In June, he underwent a back operation to relieve a trapped ligament that felt like "a dozen machine guns... shooting bullets at my spine". Most recently, his doctors have detected a problem with his prostate and are doing further tests.

Sherry endured many misfortunes in the course of his research. For seven years, he followed in his obsessively peripatetic subject's footsteps, and the effort almost killed him. In Panama, he contracted gangrene of the intestine. In Haiti under the Duvaliers, he was detained at the airport and almost arrested after customs officers found a copy of Greene's anti-Duvalier novel The Comedians in his luggage. In Liberia, he passed out from tropical diabetes and was lucky to be rescued by a Peace Corps doctor who gave him bottle after bottle of 7-Up to restore his blood sugar. Also in Liberia, a roadside thug pushed a revolver deep into his ear and permanently damaged his hearing. It did not help, on that trip, that he was still wearing an eye patch from a car accident that left him blind for six months.

Sherry's susceptibility to accident was so alarming that Greene himself discreetly intervened to prevent him visiting a leper colony in the Congo. Sherry had asked Greene's original guide, Michel Lechat, to take him there, and believed for years that he had been turned down because it was simply too dangerous.

After Greene's death, however, he found a letter from Greene to Lechat begging him to stonewall. "I am terrified of Norman Sherry going to the Congo," Greene had written in December 1984. "He caught the same dysentery as I did in Oaxaca, he had an operation on his intestines and I had an operation on my intestines, he nearly got involved in a revolution in Liberia, there were shots in the streets of Panama when he was there which in all my visits I have never heard and I fear he will catch leprosy if he goes to the Congo."

It is hard not to see something darkly, even perversely comical in the relationship between Greene, one of the most secretive and complicated literary figures of the 20th century, and the short, wiry, energetic but ultimately doubt-plagued English professor whom he chose to chronicle his life.

Sherry certainly had promising credentials: he had made his name with a pair of volumes on Joseph Conrad in which he retraced the writer's journeys and uncovered a wealth of material unknown to previous biographers.

One has to wonder, though, whether Greene chose him because he thought Sherry could do the task justice, or because he thought it would be interesting to watch him flounder in his exhaustive attempt to achieve the impossible. Sherry was well aware that Greene viewed his assignment, at least in part, as an existential game, but he was also determined to come out the winner. "When volume one appeared, he realised I was very serious in my game," Sherry said. "After he had read it a second time, he asked me: 'How did you make these discoveries? I never told you these things.'"

Sherry's maxim, from which he never wavered, was that he could find out anything as long as he dug deep enough and looked for long enough. Some of his best clues he found through close reading of Greene's novels, which he then sought to corroborate with the most disparate of sources - letters, the testimony of friends, medical and psychological records, and so on. This had been his approach to Conrad, and which had claimed 10 years of his life as he travelled from the Far East to the African "heart of darkness".

With Greene, the effort became almost overwhelming. "I'm not a slow writer," Sherry said. "I don't hang around - I move. But this was enormous. Everything was unbelievable!" Greene was a man of many secrets, and his life contained as many multitudes as the man himself - love affairs, psychic and religious breakdowns, differing accounts of the same events (including at least two separate versions of his diaries) and a seemingly unquenchable thirst for danger and experimental behaviour. Sherry refused to be daunted by any of it.

Soon after they began collaborating, for example, Sherry asked Greene about the suicide attempts he had made as a teenager. When Greene made clear that he would not talk about them, Sherry let him know that this would merely slow, not deter, him: "I told him, 'Mr Greene, you just put five more years on this book.' I then set about finding as many of his old schoolmates as I could. I tracked down 36, and two of them had kept diaries."

Sherry firmly believes that, by the end, he and his enigmatic subject became good friends. Greene made a pledge that, while he would not always answer Sherry's questions, he would never lie to him. In return, Greene expected Sherry to be unwavering in his own commitment to the truth. "If you write for me, fine," he told Sherry in one of their very last conversations. "If you write against me, fine. The truth, Norman, the truth, else I'll haunt you!"

According to Sherry, the success of the relationship was based on the fact that both held to their end of this bargain. He retains a boundless respect for Greene for allowing the chips to fall where they may - a quality he recognises as all too rare in anyone, much less a famous writer. "There are many authorised biographies where the writer had to be careful. I didn't have to be careful. I had to be truthful."

The relationship between biographer and subject began as strangely as it was to go on. It was 1974, and Sherry had travelled from Lancaster, where he was the head of the English department, to meet Greene at the author's London club, the Savile. The night before they met, Sherry ran into Greene's private priest, Father Leopoldo Duran, who he assumed was on some sort of advance spy-mission.

When Greene appeared the next day, he looked his prospective biographer over with his eerily pale blue eyes ("like a Siberian husky's," Sherry now says) and declared: "I don't like intellectuals. I don't like academics. Yes, they're interesting from the Adam's apple upwards. But they don't wear pants."

Sherry was startled, but guessed he was being tested. He said nothing. (He had at that point read just one Greene novel, This Gun For Hire, so he had reason to be less than forthcoming.) Greene remarked that it was impossible to get into Vietnam, with the war raging. He added: "There are a lot of women in my life. I'm not sure I want you writing this up."

The encounter did not seem to be going well. "He had this unusual face, and those light, light blue eyes," Sherry recalled. "He had a nice, soft-toned voice, but those eyes... they've continued to frighten me all my life. And he was taller than me, dammit." Just when it all seemed to be over, Greene suggested taking a walk to the place where he used to meet Kim Philby. As they walked up Pall Mall and into St James's Street in a light rain, Greene, without warning, broke into a sprint, ran across the road and was knocked to the ground by a braking taxi. It's hard now to know who was more stunned, Greene - who was unharmed - or Sherry.

"You almost lost your subject, Professor Sherry," Greene said as the younger man bent down to help him. And Sherry responded: "Not as bad as losing your biographer, Mr Greene." At which point Greene laughed, held out his hand, and said: "It's on then."

So the adventure began. Greene told his family that he lived in terror of his next round of Sherry interviews, but to his biographer he was unfailingly courteous and obliging. When Sherry suggested that the project would take him three years, Greene told him it would be 20, "and then some".

Sherry took this and many of Greene's other pronouncements to heart in ways that took him far from the biographer's ideal sense of detachment. In his own words, he wanted to be "cool" but could not be - especially as the worst of his tropical illnesses took hold and he realised the full scope of what he had taken on.

Greene told him to go to Texas, where a large batch of love letters to his wife Vivien was archived. So Sherry - who initially thought he'd rather go to hell than Texas - gave up his job at Lancaster and joined the faculty at Trinity University, where he now holds the position of distinguished professor.

Greene also, unwittingly, introduced him to the problem of mortality, which caused him to think he had to divorce his wife, who did not want children, and find one who did. Sherry did not go into details, but he showed me a large black-and-white photograph of the first wife, whom he plainly still adores. He ended up marrying the assistant he had hired to help decipher Greene's handwriting at the University of Texas archive. Two children duly followed - the first given the middle name Graham - but they have grown up largely without their father because of his obsessive work on the biography. This second marriage has now failed, too.

What did Greene make of all this? It's hard to know. He never ceased to support Sherry's research, even after volume one came out in 1989 (Greene died two years later). But several visitors to his homes in Antibes and Switzerland detected a certain bemusement, or perhaps something less charitable. David Lodge, who visited Greene in 1985, reported that he "seemed to derive a mischievous glee from the tribulations that poor Norman Sherry had suffered in trying to retrace his every step".

Sherry himself told an intriguing story of a summer visit he made to Greene's "lucky home" on the island of Capri ("lucky" because Greene could write with extraordinary speed and intensity there). Sherry thought he was alone there with Greene's friend Countess Cerio, only to be surprised in the middle of the night by a tall visitor who opened his bedroom door and then stole out of the house.

The next morning, Sherry asked the Countess if the intruder could possibly have been Greene, but she said he wasn't even on the island. Years later, at Greene's funeral, the Countess took Sherry aside and told him the night visitor had indeed been Greene, determined to spy on his biographer just as he felt his biographer was spying on him. "Norman Sherry is my doppelganger," Greene had told her. "I don't see why I shouldn't be his."

In one of their last conversations, Sherry told Greene that he had a child on the way. "Illegitimate, I hope," Greene said impishly. "No, no, I'm married," said Sherry. Greene again: "I'm disappointed with my biographer." The old man softened in the end, as Sherry told him he planned to give the boy the name Graham. Greene paused for a while, then said: "There's an old Graham going out of this world, and it is a good thing there is a new Graham coming into the world to take his place." It is a line Sherry has treasured ever since.

As we talked, both in his office and at the French restaurant, Bistro Vatel, where he put the finishing touches to his manuscript at his favourite corner table, Sherry revealed himself as a generous, open, highly engaging man. But he also came across as intensely fragile and vulnerable to the judgement of others. He's certainly not abandoning his literary ambitions - he has a 100-page book called The Mind of Greene, a novel and a study of the art of biography in the works - but he professes that he is most certainly, to borrow the title of a Greene novel, "a burnt-out case".

As the wine flowed, he talked with growing fervour about death, about his longtime refusal to acknowledge it and his desire, even now, to conquer it. Many of the people he has written about are now dead themselves; his book, he feels, is one way in which they can be resurrected, none more so than Greene himself.

"I spent seven years wandering and getting sick and 20 years writing... Do I think I've got him exactly? No, of course not," he mused. "But I got to know him better than I know myself. Better than the wrinkles on my own brow." It is an effort that will no doubt earn him the gratitude of generations of literary scholars. In securing Greene's immortality, however, he has half-destroyed himself.

'The Life of Graham Greene: Volume 3, 1955-1991' by Norman Sherry is published on 30 September by Jonathan Cape (£25)