IF THE MYTH OF objectivity did not continue to befuddle any discussion of the biographer's craft, we would all know by now that writing someone's life is a partial, haphazard and entirely subjective business.

We would then be more likely to ask whether a life is worth writing at all, why someone would want to write it and, by inference, whether the result is worth reading. Such thoughts are occasioned by the appearance of Gladus: Duchess of Marborough by Hugo Vickers, the second account of the life of the American heiress Gladys Deacon to appear recently. (The Face on the Sphinx, by Daphne Fielding, was published in 1978.)

For those who came in late, Gladys Deacon first entered the news in 1892 at the age of 11 when her father, a half-mad Boston millionaire, shot and killed his wife's French lover in Cannes. Gladys soon give evidence that she too had some of the family's demonic obsessiveness when she decided, at the age of 14, to become Duchess of Marlborough one day. She eventually did, although it took her another 26 years to bring it off. In the meantime, she became a celebrated beauty and was the kind of mocking flirt once considered the zenith of a girl's ambition. Even future rulers fell like ninepins. The crown prince of Prussia gave her a ring, but the kaiser made her give it back.

There is strong argument that Gladys Deacon barely warrants one study, let alone two. Dozens of equally attractive unknowns, buried under the multilayered trivia of library collections, can boast of equally scandalous pasts and something more substantial, by way of accomplishment, than (as in the case of Gladys) the ability to look alluring, chatter amusingly and marry the richest man in England.

Gladys, in short, pursued the familiar path of primrose heroines who believe their own publicity. Born comely, they accept the verdict that their worth is the sum of their physical parts. In pursuit of ever-more-perfect exteriors, they experiment ruinously -- in the cast of Gladys, with wax injections to the bridge of her nose. Having fulfilled their ignominious marital ambitions, they become predictably miserable, age quickly and develop such alarming eccentricities that they have to be carted off -- as Gladys was -- to insane asylums, mad as hatters. The fact that some of these belles have been indoctrinated since infancy with the idea that their only worth is their beauty makes their fates only marginally more interesting. Plenty of others have always known better.

What, then, was the magic of Gladys' biographical allure? Daphne Fielding's interest apparently dates from the moment she heard the name first fall from the lips of her husband, the Marquesss of Bath. Hugo Vickers, by contrast, was impelled by what seems a sincere and continuing determination to find the real woman behind the rumors.

Why Gladys Deacon should have become an obsession for a teen-age boy -- he first read about her in 1968 in the diaries of Sir Henry Channon -- for the next decade of his life, is an unanswered question. Nor can one understand how Vickers could have been patient enough to endure, through 65 visits, the opinionated utterances of an old lady, especially after she "launched into one of the most fluent onslaughts on my character that I have had the misfortune to endure." Vickers provides no clues. He knew, however, that another biographer was at work, causing him to apply himself in earnest. In the Deacon biographical sweepstakes, Vickers wins hands down. His study is far fuller, better detailed and more thoroughly researched than Fielding's earlier one.

His book is also more persuasive. Gladys Deacon's father is portrayed by Fielding as a sweetly reasonable man pushed past his limits by a flighty wife. Vickers argues, more plausibly, that Edward Deacon was pathologically jealous and early on showed signs of the madness which later claimed him.

In those not so distant times, French lovers inept enough to be found with another man's wife, deserved to be shot. Deacon was let off in the French courts with a year's imprisonment (even that sentence aroused groans of sympathy) and was subsequently granted custody of his three eldest children. oFielding dismisses this fact with a sentence to the effect that Mrs. Deacon shortly regained custody. In fact, Gladys was removed from her mother at the age of 12 and forced to live 3,000 miles away from her for three years. Vickers rightly emphasizes the anguish of that separation.

Fielding's superficial account of the romance between the emerging art historian Bernard Berenson and the young flirt is similarly incomplete. To Fielding, that friendship was a mere flirtation. Vickers establishes that Berenson had only just married Mary Pearsall Smith when, in 1901, he decided he'd much rather have had Gladys.

However, the newer study is not without its shortcomings. Vickers seems to want to set Gladys' most trivial judgments in the concrete of repetition. We learn, for instance, of her husband's cousin, Winston Churchill, that, "He was the right man for the time because he was showy and that's what they wanted," at least twice. Vickers' chatty style (he talks about Edward Deacon's keeping "a cautious low profile") is particularly ill-suited to the material.

The largest problem with this biography, as with the earlier study, is the triviality of its insight. After having chronicled her life with devoted care, Vickers concludes lamely that, "Charm is a dangerous quality, and it is hard to keep the champagne sparkling twenty-four hours a day," and, "She paid a high price for her coronet." There is, however, a kind of poetic justice about that.