"Your threat to leave Knopf after this current contract is fulfilled leaves us far from intimidated," Robert Gottlieb wrote Roald Dahl in 1980, after a long association that saw "James and the Giant Peach" and "Charlie and the Chocolate Factory" become two of the best-loved and best-selling children's books in history. "You've managed to make the entire experience of publishing you unappealing for all of us. . . ." Mr. Gottlieb continued: "To be perfectly clear, let me reverse your threat: Unless you start acting civilly to us, there is no possibility of our agreeing to continue to publish you."

"Civilly" was not an adverb of Dahl's grammar -- "boorishly," perhaps, and "prodigally," "piggishly," "generously," "childishly" and "enchantingly." As his editor, of course, Mr. Gottlieb knew it. How many editors today, out of sheer self-respect, would dismiss an author who ran up royalties of about $5 million a year?

Dahl stomped over to Farrar, Straus & Giroux for four successful books, huffed along to Viking for one more and died three years ago, leaving behind an uneasy reputation. He had written virulently about Israel and the Jews, had broken with his fellow literati -- who never accepted him as one of their own -- by denouncing Salman Rushdie instead of Rushdie's death sentence; and had distilled some odd, dark flavors in his writing that put off children's book specialists, though not children.

Visiting Dahl's grave in Great Missenden, England, Jeremy Treglown found it garlanded with plastic toys and a large onion. He has written the onion. His "Roald Dahl: A Biography" is as fumy, cantankerous and uneven as Dahl himself; sometimes mean, often funny and yet, after layers of peeling, offering us a fair-minded and even generous portrait.

Mr. Treglown gives a concise but effective account both of the children's books and of the clever, grisly short stories that Dahl wrote in the 1950s. Without straining, he portrays a Dahl who remarkably resembles his writing: a 6-foot-6 child of a man, with a need to bring off marvels and slay dragons. All too often both the marvels and the dragon-slaying were performed upon his friends and family, who lived in a steady din of feats and prowess.

As a bachelor in Washington, he painted the genitals of the rhinoceroses on the Q Street Bridge. After telling his children bedtime stories of a giant who prepared dreams and blew them through the windows, he put up a ladder outside the nursery, climbed it and rustled their curtains.

Dahl was the son of a wealthy Norwegian ship broker who settled in England and died when he was young; he was raised by his mother and several doting sisters. He joined the Royal Air Force in World War II, and almost immediately crashed in the Egyptian desert after running out of fuel. He would embroider the embarrassment into a tale of heroic action; on the other hand, once recovered from severe injuries, he became an authentic flying hero.

Assigned as an air attache in Washington, his real mission was to be a --ing example of British military prowess to capital circles whose Anglophilia Churchill counted as a main war resource. He hung out with Ian Fleming, C. S. Forester, Martha Gellhorn, Ernest Hemingway; and made love to a number of glamorous women including, by his account, Clare Boothe Luce.

Most important, he became the close friend and lifelong protege Charles Marsh, a wealthy oilman with a taste for fantasy and game-playing equal to Dahl's. Marsh's money helped him keep going when he returned to England to write; his and his wife's friendship helped steady Dahl's stormy marriage to the actress Patricia Neal.

It was a remarkable marriage and it serves as the appropriate centerpiece of Mr. Treglown's biography. It lasted 30 years, and divided with the drama of a Dahl tale into two sharply contrasting parts. For the first half Neal was the great figure: a Broadway and Hollywood star, beautiful and loved as well as courted by an array of glamorous friends. She had the money and the fame; Dahl, whose short stories had won him modest shares of each, had not yet begun the children's books.

The inequality made him all the more flamboyant and imperious; he dominated or overturned every dinner table. Neal, by Mr. Treglown's absorbing account, loved and understood him enough to keep things going. He managed the money; she made his breakfast and lunch and bathed the babies -- they had five -- before going to work.

Twelve years later, in 1964, she had three disabling strokes, which put an end to the career, the glamour and her ability to care for her family. It seems to have been a spectacular if faintly queasy liberation for Dahl.

With his first children's books catching fire, he was the moneymaker. At home he became the immensely present father, and the fiercely authoritarian supervisor of Neal's long convalescence. Like Willy Wonka of the chocolate factory, as Mr. Treglown points out, he could be the great magus and panjandrum, both in fantasy and fact.

Mr. Treglown, a former editor of the Times Literary Supplement, gives a full account of Dahl's arrogance, intemperance and bullying qualities. His story of Dahl's tantrums with his publishers, which grew worse as he grew more successful, virtually turn him into one of his own fairy-story monsters. Yet Dahl's monsters had a comic charm; and so does Mr. Treglown's Dahl.

Eventually -- 18 years after the strokes -- Dahl left Neal for Felicity Crosland, with whom he had been having an affair for several years. Mr. But Treglown talked at great length with Neal, and more briefly with two of their daughters. (The second Mrs. Dahl and a third daughter, who is also writing a biography, did not cooperate.)

What he found was a mixture of gratitude, anger and awe at the memory of this human whirlwind, who blew debris along with dreams in through their windows, and blew them promptly out again through their door.

Title: "Roald Dahl: A Biography"

Author: Jeremy Treglown

Publisher: Farrar, Straus & Giroux

Length, price: 322 pages, $25