In 1944, the German artist Joseph Beuys, then a pilot in the Luftwaffe, was shot down in the Crimea and rescued by Tatar tribesmen who saved his life by wrapping his burned body in fat and felt. This event - which is probably fictionalised, if not entirely fabricated - provided Beuys with both the central element of his self-mythology and the iconic materials for his later works of art.

Four years earlier, Roald Dahl, who had a similarly buccaneering approach to the truth, was also involved in an air crash during his wartime stint as an RAF pilot. On the way to join his squadron in North Africa, he lost his bearings and bungled a forced landing in the desert in Libya, destroying his Gloster Gladiator and incurring severe head injuries. He swiftly metabolised this event into a more palatable fabular version in which he was gunned down during enemy action, and as such it formed the foundation of his embryonic literary career.

The tendency to recycle and reassemble certain pivotal moments continued throughout Dahl's life, both in his published fiction and
in his self-appointed role as anecdotalist and gossip. The title of Donald Sturrock's whopping authorised biography alludes to this compulsion, but also gives it a characteristic gloss - "storyteller", after all, is a good deal more agreeable a term than its close relative, "liar".

It's a distinction that would have meant little to Dahl himself, who thrived on riling and infuriating establishments of every kind. Born of Norwegian parents, he never felt entirely accepted by the English and proved a little too iconoclastic to merge comfortably into Repton, the Derbyshire public school whose brutalities he minimised, if anything, in his sort-of autobiography, Boy.

In 1942 his career in the RAF took him to Washington, DC, where his rollicking plain speech and startling good looks made him popular with the then president, Franklin D Roosevelt, as well as a bevy of influential society ladies. (A contemporary once described Dahl as "one of the biggest cocksmen in Washington"; Sturrock favours the epithet "the handsome air attaché", which presumably amounts to the same thing.)
After dipping a toe in both spying and the movies and securing himself a glamorous wife, the American actress Patricia Neal, he returned to what his daughter later called "the Valley of the Dahls": the region of Buckinghamshire where much of the remaining clan had settled. There he set about transforming himself into a full-time writer, first with the clever, nasty stories that reached their apotheosis in Kiss Kiss and Switch Bitch, and then with the run of irreverent and gleefully brutal children's books that made him fabulously rich.

Sturrock is an affectionate, even tender, biographer - unsurprising, perhaps, given that he is writing at the invitation of the surviving family. He deals with the travails that afflicted the Dahls during the 1960s - the injury of one child, the death of another, followed by Patricia's almost fatal stroke - with even-handed sensitivity, showing a man both indomitable and so calloused by crisis as to be almost cruel.

The author's skill at ferreting out the minutiae of Dahl's life is less effective when it comes to the books themselves, which he tends to treat primarily as a repository for autobiographical gleanings. As a result, Sturrock blurs features of Dahl's life with those of his prota­gonists, fusing him with almost everyone he ever wrote about, from the airmen of his early stories to "the top-hatted chocolate magnate" Willy Wonka and the eponymous BFG (they wore the same Norwegian sandals). This approach leaves the value and context of his literary work oddly unscrutinised, a problem only intensified by the obsessive attention the book gives to his whumping fallouts with almost every publisher in London and New York.

Where Sturrock does succeed is in showing Dahl's unstinting creativity and the attention he paid to the darker aspects of humanity. Deeply sensitive as a child, he all too frequently assumed the role of bully as well as defender of the underdog in later life, and plenty of his stories were turned down because their content was excessively grim or macabre ("a little too unpleasant for our general readers", as the fiction editor of the New Yorker once put it).

While not enough is made of the deep seam of misogyny that runs through much of his adult work, the Dahl that lopes from these pages is undoubtedly a giant of a kind: possessed of an overpoweringly fertile imagination, his brain gusting like a whizzpopper, working his way through the outer reaches of man's capacity for goodness as well as malice.

Storyteller: the Life of Roald Dahl
Donald Sturrock
HarperPress, 448pp, £25