But there is, it seems, one taboo topic for the 56-year-old: herself.
Yet Winfrey's refusal to be interviewed seems to have inspired, rather than deterred, Kelley. She interviewed 850 sources for "Oprah," including major "gets," such as Winfrey's father, and used the thousands of interviews the rags-to-riches cultural icon has granted over the years to paint a portrait that becomes less flattering as Winfrey's fame and influence grows.
The result is an impeccably researched and well-organized look into the unlikely life of a self-made woman who used her poor upbringing as a steppingstone to build an astounding media empire. Winfrey's net worth is currently valued at $2.4 billion.
"Oprah" is, in many ways, a brave and audacious book. Circumventing a highly controlled Oprah-sphere that, for more than a decade, has required all employees to sign nondisclosure agreements, Kelley attempts, and largely succeeds, in piercing the veil of one of the most powerful, influential and wealthy women on the planet.
"Oprah," the book, methodically traces the rise of Oprah, the woman, in fast-moving chapters about her upbringing, career, romances and causes, all of which highlight her many charms and contradictions.
It's a fascinating portrait of a woman who is willing to FedEx horses from Indiana to Hawaii but will freeze out friends who say anything disparaging about her, a woman who built a $40-million school for disadvantaged girls in South Africa but didn't adequately vet its staff; a woman with a male romantic partner who spends most of her free time with gal pal Gayle King; a woman who stumped for Barack Obama but may have had a hand in electing George W. Bush.
Kelley begins with Winfrey's 1984 debut in Chicago, where the affable if overweight talk show host shook up daytime TV with titillating, often sexually oriented, subjects that blurred the line "between discussion and confession, interviews and self revelation."
Such a formula worked from the get-go, as Winfrey acknowledged on-air that she had been an incest victim when she was 9. Later, saved-for-ratings-week revelations included admissions about abusive lovers, her teenage sexual promiscuity and the fact that she had given birth, at age 15, to what most likely was her uncle's child. The child died after about a month.
Winfrey herself was an illegitimate child. She is the eldest of three children her mother had with three different men she never married. Both of Winfrey's half-siblings have since died — her sister of a drug overdose, her brother of AIDS.
She is not close to her mother and has had a tumultuous relationship with the man who, although not her biological father, raised her as if he were. This situation has likely contributed to her unbridled quest for success and recognition.
It would be hard to find a better poster child for overcoming adversity than Winfrey: a homely African American girl born in the segregated South in 1954, so poor, she has said, that her only pet was a cockroach.
As she has demonstrated repeatedly, bad situations haven't been a hindrance. It was through affirmative action that she entered the media in an effort to become "the black Barbara Walters," landing jobs at a Nashville radio station and then a Baltimore TV station, where she botched her gig as co-anchor of the nightly news but bedded the future star of "Entertainment Tonight," John Tesh.
Kelley's account of Winfrey's early, pre-Chicago years provides many of the book's most enlightening — and surprising — moments, since they reflect some of the few times her subject has actually failed.
But the full story is equally compelling, as Winfrey evolves from sex-abuse victim to international media mogul capable of turning unknown books into bestsellers and little-known talking heads such as Drs. Phil and Oz into successful talk show hosts in their own right.
If there's a downside, it's that, as Winfrey's empire grew, she began to restrict her own personal spontaneity and openness.
"Oprah" addresses that also, in a well-orchestrated whirlwind of a book that is as fast-paced, cunning and alluring as Winfrey herself. Kelley might not have spoken to her subject, but she gets her just the same.