The tall, handsome man cut a tragic figure as he wandered the corridors of New York's Lenox Hill Hospital on a cold day in December 1960. With shoulders hun-ched, he shuffled through the wards like a ghost. Hours earlier, his four-month-old son, Theo, had been struck by a taxi on Madison Avenue. The doctors' report was blunt; the child's skull was fractured in several places, and there was fluid on the brain. It was likely he would die.

For days, the man and his wife sat side by side, watching over their tiny baby in a rare show of unity in an otherwise tempestuous relationship. It was a time which, many years later, the woman would say, "was, paradoxically, one of the most married times in our life together".

So, a few weeks later, it was nothing short of a miracle when the couple were able to take Theo, who had recovered, back to their Manhattan apartment. His father must have felt grateful that fortune was smiling on him for once. What he didn't know then was this moment of grace would be a rare high point in a life struck by tragedy, time and time again.

When Roald Dahl carried his young son out of the hospital in January 1961, he was, at 44, a moderately successful short-story writer. His wife, however, Oscar-winning actress Patricia Neal, was a superstar and she served as both breadwinner and celebrity in the household. Yet within just a few years, their roles would reverse in such a dramatic fashion, it was as if Dahl had crafted the narrative of their lives himself. After a series of personal tragedies, including three near-fatal strokes, Neal's career collapsed, while Dahl became one of the most celebrated writers of his time.

During his 45-year career, Dahl delivered an exhaustive body of work spanning adult short stories, Hollywood scripts and children's fiction; his writing characterised by its sense of humour and artfully dark characters. But while children loved novels such as Charlie And The Chocolate Factory, The Witches and Matilda, his adult fiction fared less well, a thorn that pricked Dahl's pride throughout his career. Critics charged his stories with demonstrating misogyny and racism, but for fans, Dahl was one of the most magical minds of the 20th century. "I can still see him now," said his daughter Ophelia, many years after his death, "leaning against the wall of our bedroom with his hands in his pockets, looking into the distance, reaching into his imagination."

Roald Dahl was born on September 13, 1916 in Llandaff, Wales, to Norwegian parents, Harald and Sofie. He was an only son, with two older sisters, Astri and Alfhild, and a younger sister, Else. Harald owned a successful shipping brokerage, and the Dahls led a privileged life. But the idyll was short-lived. In 1920, when Roald was just three, Astri was struck down with appendicitis and died, aged seven. Two months later, Roald's father, Harald, passed away after a bout of pneumonia, although some have suggested that a broken heart was to blame. The themes of premature death and tragedy were etched into Dahl's family template.

At school, Dahl was academically weak - towards the bottom of his class in some subjects - but he'd inherited his father's passion for keeping a diary and, from the age of eight, he'd write daily in a secret journal. "I used to put it in a waterproof tin box tied to a branch at the very top of an enormous conker [chestnut] tree in our garden," he once said, "to make sure none of my sisters got hold of it." At 18, instead of going to university, Dahl got a job as a salesman at Shell Oil. Within a few years, the company assigned him to a post in Tanzania. It was 1938, and a world war loomed. Mussolini's Italy already ruled Libya and had invaded Ethiopia, so when Dahl, 23, heard the RAF was recruiting pilots in nearby Nairobi, he enlisted.

Years later, Dahl - the eternal fabulist - would make much of his war efforts, but the truth was his involvement was comparatively minimal. Although he saw action in Greece and Egypt, a crash early in his RAF career left him with head injuries, which caused him severe headaches and frequent blackouts. It wasn't long before he was sent back to Britain.

While Dahl may not have had a fierce intellect, he possessed spectacular charm, wolfish good looks and steely social ambition - a potent combination that gave him access to some of the UK's most influential circles. It was within this world he met Harold Balfour, the Under-Secretary of State for Air, who, impressed with Dahl, sent him to Washington in 1942 to serve as an assistant air attaché at the British embassy. The US had just joined the war effort and, although the two countries were allies, the British intelligence services were not above keeping tabs on the Americans. A young, handsome man like Dahl was exactly what they needed.

Dahl was not officially a spy, but he was encouraged to get close to well-placed people - especially women. "Girls just fell at Roald's feet," said Antoinette Marsh Haskell, the daughter of Dahl's closest American friend. "He was very arrogant with them, but he got away with it. I think he slept with everybody on the east and west coasts that [was worth] more than $50,000 a year." One conquest was Clare Boothe Luce, an attractive woman 13 years Dahl's senior and wife of Henry Luce, publisher of TIME magazine. Dahl was reportedly so vigorously entertained by Mrs Luce that he complained to a friend: "I'm all fucked out. That goddamned woman has absolutely screwed me from one end of the room to the other for three goddamned nights."

Still, cavorting with America's high society paid other dividends, and it is believed this is how the budding author got his entree into the literary world. At just 25, Dahl, who had only ever written a couple of short stories, was invited to Hollywood by Walt Disney to adapt a film script for one of his stories about RAF "gremlins" - mythical supernatural forces blamed for mechanical problems that struck RAF missions. While the script never became a film, Disney published The Gremlins in 1943 as a picture book, and Dahl's writing career was underway.

After The Gremlins, Dahl based himself in New York, where he wrote short stories that appeared in some of the US's most influential magazines, including The Atlantic Monthly and The New Yorker. Although he was earning enough to carve out a career as a writer, he was not yet a celebrated name. But Patricia Neal certainly was. The enchanting actress from Kentucky was just 25, and considered one of Hollywood's most exciting talents, when she first met Dahl at a party thrown by mutual friends in 1951. Although successful, she was heartbroken after ending a three-year romance with Gary Cooper. But Dahl proved more than capable of replacing the matinee idol and, on a stifling hot day in July 1953, they married at the Trinity Church in New York. Neal wore pink chiffon, while Dahl tore out the lining of his suit so he could cool down.

Neal was smitten, but friends were concerned about Dahl's tyrannical hold over her. Composer Leonard Bernstein, a close friend of Neal's, told her she was making the biggest mistake of her life. And he may have been right. Within six months of marrying her, Dahl - apparently disappointed with Neal's lack of domestic talent - petulantly announced that he wanted a divorce. But any friction was later forgotten when, in 1955, they welcomed a daughter, Olivia, followed two years later by Tessa. So began a blissful period. Dahl's first published book of short adult fiction, Someone Like You, sold a record number of copies for its genre. The family coffers swelled and they purchased a huge farmhouse (which was later named Gipsy House) inBuckinghamshire, an hour from London. But the author was still unsettled. He felt overlooked by critics and, many believed, was rattled by his wife's superior career.

The seeds of his greater destiny were sown in his children's rooms, where Dahl would conjure bedtime stories for Olivia and Tessa. In 1961, just a few months after his infant son, Theo, was released from hospital, Dahl submitted a draft to his American publisher of a story about an orphan who travels across the world on a giant peach, famously saying: "What the hell am I writing this nonsense for?" as he finished the final words. The "nonsense" was James And The Giant Peach, a children's classic that still sells thousands of copies every week worldwide. He was also scribbling away at another children's story, Charlie And The Chocolate Factory.

But before Dahl could enjoy the success of either book, he'd have to face one of the biggest tragedies of his life. In 1962, Olivia, then aged seven, contracted measles. When she slept for 24 hours straight, a concerned Neal called the doctors. But it was too late. Olivia went into a coma and died in hospital of measles encephalitis. Dahl was inconsolable and tried to escape by burying himself in work. He began to drink heavily and upped his dose of barbiturates, which he'd long taken for back pain. When little Tessa became fractious, he gave them to her, too. However, amid all the turmoil left by Olivia's death, Neal and Dahl achieved incredible success: Neal won the Best Actress Oscar in 1963 for Hud, while Charlie And the Chocolate Factory, which was released in the US the following year, sold 10,000 copies in its first month.

But history had taught Dahl not to trust good fortune. While still grieving the loss of Olivia and anxious about Theo, who had severe learning difficulties as a result of the car accident, Dahl was sent reeling again when Neal suffered a series of strokes in February 1965. Three weeks later, when she finally awoke from her coma in hospital, her first words were: "Who are the people in this house?" And when she was eventually allowed to go home, she couldn't talk or move her right side, and was told she'd have to relearn many of her communication skills.

"Everything was changed," admitted Neal. "It was sad because our marriage, it had been really good ... and then when I got ill, everything changed in the end. Everything was turned upside down."

Dahl supported the family - which had now grown to six with the birth of two more daughters, Ophelia and Lucy - with a successful screenplay for the 1967 James Bond movie, You Only Live Twice, as well as royalties from James And The Giant Peach and Charlie And The Chocolate Factory. But supporting a brain-damaged son and a troubled marriage took a toll on Dahl, and he sought an escape.

Diversion came in the form of Felicity Crosland, an attractive divorcee in her mid-30s, whom Dahl had met through Neal. The two women had become friends through Crosland's work as a set designer on the coffee commercials which Neal starred in after her recovery. It wasn't long before Crosland and Dahl, who nicknamed her "Liccy", became lovers. Neal didn't find out about the affair for many years, but Tessa knew about it early on after overhearing her father speak passionately on the phone to "Liccy" one evening. When she confronted him, he is said to have replied: "You've always been a nosy little bitch. I want you to get out of this fucking house now." Tessa didn't divulge her secret until several years later when, in 1983 - after 30 roller-coaster years of marriage - Neal and Dahl divorced.

Wounded by his betrayal, Neal would not attempt reconciliation for many years while Dahl, according to neighbours, had never been happier. "All the years that I first knew Roald I never saw him peaceful," noted one. "If there's such a thing as inner peace, that came with Liccy." With a new partner and a new illustrator for his books - Quentin Blake, a quiet man whose bristly, comical illustrations perfectly tempered Dahl's offbeat characters - life was looking up. In 1982, his tale of a big friendly giant, The BFG, was published to great acclaim. The following year, he married Crosland and won the prestigious Whitbread Prize for his new novel, The Witches. Between 1984 and 1988, he published two autobiographies - Boy and Going Solo - as well as Matilda, the phenomenally successful children's book.

But these latter years were plagued by controversy. Dahl had already been in hot water over what some felt was the politically incorrect depiction of his Oompa-Loompas in Charlie And The Chocolate Factory. In Dahl's original version of the novel, the Oompas were a tribe of black pygmies imported from "the deepest and darkest part of Africa". There was worse to come. In a 1983 interview, Dahl made a number of anti-Semitic comments. The public was appalled and some book traders announced they'd no longer stock his work. But Dahl was unrepentant and added further fuel to the fire with another verbal attack on Jews.

In 1985, the ageing writer was in hospital with bowel cancer and, although the surgery was successful, he was left weak and hobbling on walking-sticks. Still, Dahl tried to make a comeback as a writer of adult literature with two pornographic tales: Princess Mammalia and The Princess And The Poacher, which were printed in 1986. In 1990, as The Witches and Danny The Champion Of The World were made into films and his novels sold almost two million copies in the UK alone, he received a letter; it was Neal, asking for a reconciliation. While they did reunite for Theo's 30th birthday party, it was the last time Neal saw him alive.

On a chilly afternoon on November 23, 1990, Dahl died of a rare blood disorder at the age of 74. He was buried on a hillside opposite his beloved Gipsy House, next to the grave of his stepdaughter, Lorina, Crosland's daughter, who'd passed away eight months earlier from a brain tumour. In a newsletter to fans, Dahl intimated that something was wrong, and in doing so, provided a succinct - if modest - summary of his life. "I've been a bit off colour these last few months," he wrote, "feeling sleepy when I shouldn't have been and without that lovely old bubbly energy that drives one to write books and drink gin and chase after girls."