One thing is certain about Kitty Kelley's latest biography – you won't see it recommended in Oprah's Book of the Month club. The queen of confessional television is, according to the queen of the celebrity hatchet job, "a biographer's gift", specifically because she's "choked with secrets". Among the supposed skeletons that Kitty has clawed out of the Winfrey cupboard is the claim that the presenter was not raised dirt poor (as she so often claimed), that Vernon Winfrey is not her biological father, and that her relationship with Gayle King is more intimate than mere girly friendship. Kelley claims to have conducted almost 850 interviews for this latest unauthorised biography, about the same as for Frank Sinatra – His Way, published in 1986. In it, she portrayed Ol' Blue Eyes as a misogynist and a mobster. Even though the furious crooner sought to block publication, he had to withdraw his €2m lawsuit, and, by default, turned it into a massive PR success. But no-one was left in any doubt as to the Sinatras' attitude to Kelley: his daughter Nancy said, "I hope she gets hit by a truck."

That the name Kitty Kelley and 'poison pen' are synonymous results from those previous five unauthorised biographies, that began with Jackie Oh! That was back in 1978 when Kelley rummaged through the former First Lady's dirty laundry, detailing her alleged mental illness and JFK's serial infidelity. While the big picture of her subjects is accepted as broadly accurate, she is no Clare Tomalin or Richard Holmes in the biographer stakes. The salacious details that sell her books frequently don't stand up to scrutiny. Famously, she claimed to have interviewed Peter Lawford for the Kennedy book – even though he had died 12 days beforehand. "It's clear that she is no meticulous historian who nails down her facts with airtight precision," opined Slate magazine. "People read Kelley for the same reason they read the National Enquirer: the taboo."

And yet, two facts remain true, so far: she has never been successfully sued, nor has she been legally made to publish any retractions. As for her personal life, Kelley manages to keep her background fairly private, although she came under the scrutiny of George Carpozi Jr in 1999 with his Poison Pen: The Unauthorized Biography of Kitty Kelley. Naturally, it didn't sell as well as Kelley's own books – gossip about an icon is always going to attract more attention than a woman who once said "Hell, for a million dollars I'd write about Donald Duck."

Born in Washington 68 years ago, Kelley is the daughter of a lawyer, and, according to Carpozi, a heavy-drinking mother. In her teens, the petite, blonde, blue-eyed 'Lilac Princess' was voted 'Friendliest Girl' at high school. After graduating with a BA in English, she worked as press secretary for Senator Eugene McCarthy, before joining the Washington Post in 1969. But it was only when she went freelance and turned one of her features – The Glamour Spas – into a full-blown gossipy book about celebrity retreats, that a publishing legend was born. Publisher Lyle Stuart commissioned the bestseller Jackie Oh! and Kelley found her niche producing what detractors dub 'Kitty litter'. She later fell out with Lyle, and the disgruntled publisher became a source in Poison Pen, claiming that when she was asked how she knew of one of Jacqueline Kennedy's intimate conversations, she admitted, "I made it up."

A €150,000 advance from Simon & Schuster followed for The Last Star, her 1981 tome on Elizabeth Taylor, and then came her equally unauthorised book on Sinatra, one of the biggest-selling biographies of all time. Given her subject matter, there may be some substance to Kelley's claim that there was a mob contract out on her. There was certainly a high degree of security prior to publication of her 1991 unauthorised biography of Nancy Reagan, with Kelley claiming that her manuscript was read under guard by only a carefully chosen few. Next came 1997's The Royals, revealing stuff that was hardly of much surprise to most British royal watchers, such as the Windsors' attempts to conceal their German ancestry or that the late Princess Diana once tried to shove her stepmother down the stairs. That it was never published in the UK points to the stiffer libel laws there than in the US, where the burden of proof lies more heavily on the plaintiff to demonstrate a damaged reputation. The Family: The Real Story of the Bush Dynasty, was published in the run-up to the 2004 presidential election. It claimed, among other things, that Laura Bush was a former drug dealer.

This time around, Kitty may have overstepped the mark by digging her claws into the revered Winfrey, whose powerful friends in showbusiness, such as Barbara Walters, Larry King, and even David Letterman, refuse to interview her.

As to the accusation that she simply rehashes reports by other writers, a former Reagan biographer says: "If Kelley ever wants to quit writing, she can get a job retreading old tyres."

Hero or Villain? Kitty Kelley

High: Fearlessness in digging her claws into revered celebrities and puncturing carefully manufactured images

Low: Innuendo and gossip presented as fact (although many of the ex-employee sources in the Winfrey bio could not be named because of strict confidentiality contracts)