US biographer Kitty Kelley, known for dishing dirt on celebrities, is to chronicle the life of Oprah Winfrey.
Winfrey's show is seen by 30m people each week in the US
Kelley, whose previous subjects include the Bush family and Frank Sinatra, said the chat show host "fascinated" her.
"There has been no other person in the 20th century whose convictions and values have impacted the American public in such a significant way."
Kelley's biography of Nancy Reagan was called "encyclopedically vicious" by the New York Times in 1991.
Kelley made her name with the 1978 publication of Jackie Oh!, which detailed President John F. Kennedy's womanising and the former first lady Jackie Kennedy's battle with depression.
In her 2004 publication The Family: The Real Story of the Bush Dynasty, Kelley alleged that President George W. Bush had snorted cocaine at Camp David while his father was president.
Kelley plans to interview hundreds of sources for the book
That assertion was credited to Sharon Bush, who was once married to the President's brother Neil. Ms Bush later denied making the allegation.
A famously vigorous researcher, Ms Kelley is said to have sent her husband out on daily trawls through Elizabeth Taylor's rubbish bins.
Her 1997 book, The Royals, a scandal-packed account of the lives of the Windsors since World War I, was never published in the UK, which has stronger libel laws than the US.
Ms Kelley was even warned by her lawyers not to speak to British reviewers for fear of legal action.
The Oprah Winfrey Show, watched by 30m viewers each week in the US alone, has been America's highest-rated talk show for 18 consecutive seasons.
Over the past 20 years, the programme has included a series of high-profile guests, while Oprah's book club has had a significant impact on literary sales in the US.
In addition to her TV career, Winfrey, 52, is an Oscar-nominated actress and a magazine publisher.
In 1996, Life magazine called her the most influential woman of her generation.
Kelley's biography will be published by Crown, a subsidiary of Random House. A date has yet to be announced.