OPRAH! By Robert Waldron St. Martin's. 201 pp. $13.95 EVERYBODY LOVES OPRAH! Her Remarkable Life Story By Norman King. Morrow. 222 pp. $14.95

Her first name is Harpo spelled backwards. She's part Hattie McDaniel, Tammy Faye Bakker, Sophie Tucker and Sojourner Truth. But mostly she's self-invented -- a sassy and offbeat talent for the barb and rhubarb that have catapulted her into a television phenomenon. Oprah Winfrey -- an obese black Cinderella who had to roll out of her limo at the 1986 Oscar ceremonies because her dress was splitting -- has proven to be so popular that her syndicated talk show is expected to gross $125 million in the 1987-88 season, with Winfrey reportedly pocketing $31 million of the pie.

Quicker than you can hum "Pennies From Heaven," two unauthorized biographies have popped up to celebrate her ascent to stardom. Both books are based on the countless on-the-record Winfrey interviews to the press and transcripts of her program. Robert Waldron, author of "Oprah!," and Norman King, author of "Everybody Loves Oprah!," use so much of the same material that their books emerge as Tweedledee and Tweedledum -- good-natured cheers from the peanut gallery.

The word "unauthorized" implies that one is about to plop down with a keg of literary dynamite, but Winfrey has been so open and honest about her life that she has already knocked the wind out of the authors' sails. Born out of wedlock in 1954 -- King says on Feb. 1, Waldron says Jan. 29 (all references consulted agree with Waldron) -- Winfrey has had a life that stirringly confirms the American dream, so what's not to celebrate?

First the obstacles: As a child she was shuttled back and forth among her relatives. While living with her grandmother in Mississippi, Winfrey suffered severe, sometimes daily beatings. ("It's what Richard Pryor described as the 'loneliest walk in your life' -- to get your own switch!") While living with her mother Vernita in Milwaukee, she was raped at age 9 by her cousin and similarly abused in her teens by her uncle. But her father Vernon in Nashville was a lovable disciplinarian, expecting nothing less than academic excellence, respect and complete obedience. ("Listen, girl -- if I tell you a mosquito can pull a wagon, don't ask me no questions. Just hitch him up!")

The rest of the story is history: Miss Black Tennessee, news reporter, talk show hostess, queen of caftans, Oscar nominee for "The Color Purple," syndication success story and this year's Emmy winner for Outstanding Talk Show Series and Outstanding Hostess. Both authors agree her show is more cathartic than cerebral, and both liberally quote the now famous Oprah-isms, from the profound ("Luck is when preparation meets opportunity") to the rowdy (to Sally Field: "Does Burt sleep with his toupee on or off?" to the downright tasteless. But when Winfrey gets too far out of line, there's always someone to knock her back a couple of notches. Once when Winfrey observed it might be awfully tough for a woman with no hands to put on a dress with 20 buttons, the handicapped woman riposted, "Yeah. Probably not unlike you trying to get into pantyhose."

King's "Everybody Loves Oprah!" is the more polished of the two, with fascinating, lengthy comparisons and contrasts with Phil Donahue's work, and a solid library clip-and-cut job. When it comes to legwork, however, King comes up short: It's obvious that he didn't even attempt to interview Winfrey's doorman. Yet King knows an awful lot about television, from its advertising revenues to its force as a cultural phenomenon.

On the other hand, Waldron, author of "Oprah!," traveled to Mississippi to Baltimore to Nashville, interviewing Winfrey's colleagues, teachers and fellow beauty contestants. But his prose style leaves much to be desired; it's rushed, awkward and often sophomoric.

Still, the latter book is preferable on a sheer entertainment level. Waldron is honest enough to attribute to the various magazines and newspapers while quoting them; King merely includes an all-inclusive bibliography at the end. And Waldron provides a you-are-there, behind-the-scenes glimpse of the Winfrey show and doesn't make short shrift of the star's love life, from Winfrey's high school boyfriend Anthony Otey to her current beau, actor-model Stedman Graham ("six feet six of terrific!"). Besides, the photos are a hoot. The reviewer is the author of "The Soap Opera Encyclopedia" and "Guiding Light: A 50th Anniversary Celebration."