GIVEN A CHOICE between an authorized or an unauthorized biography, book publishers invariably favor the proposal for the authorized version. After all, the reasoning goes, the authorized work will benefit from access to the subject, who probably would tell his associates not to cooperate with an unauthorized project. Besides, in the case of Lee Iacocca or Chuck Yeager, people want to read their story.

Yet controversial figures like J. Paul Getty, Jacqueline Onassis, or New York Parks Commissioner Robert Moses are not about to tell the entire truth about their lives, much less allow it to be published under their names. Very often, in any case, tycoons, celebrities, and politicians have little insight into how they achieved success. These two books on Greek shipping magnate Aristotle Onassis prove that whether a book is authorized or not is largely irrelevant: what counts most is the quality of the writer and his determination to get at the truth.

While not strictly an authorized work, Ari, by British journalist Peter Evans, began that way, with Onassis enlisting him to write an official biography or autobiography. The account of Evans' first meeting with Onassis in January 1968 makes some of the most fascinating reading in the book:

"His eyes were hidden by the dark glasses which were his trademark, but I knew they would be alert, speculating, not without humor. He wore a conservative blue suit with a monogrammed white silk handkerchief in his top pocket. Only his hands surprised me. Dark and hard-looking, to the touch they were soft as a young girl's."

Until Onassis married Jacqueline Kennedy in October of that year, Evans continued to interview him in London and Paris -- drinking at Maxim's, strolling the streets late at night with a pretty girl from Madame Claude's call-girl stable, sometimes with John W. Meyer, Onassis' aide, or Alexander, Onassis' son, tagging along. Onassis instructed his friends and business associates to cooperate with Evans as well.

Before deciding not to go ahead with the project, Onassis saw Evans one last time in 1974. The next year, Onassis died, and Evans was free to write the book as he saw fit, with all the warts exposed. Yet the course had been set. He continued to rely on Onassis' version of many events without probing behind them.

When a man holds a mirror up to himself, it is not only self-serving, it can be confusing. Because the stories don't mesh with reality, they often don't make sense. Thus Evans' book refers to a 1954 Justice Department indictment of Onassis and quotes his public response -- without ever telling the reader what the indictment said. (Onassis allegedly had been violating a law that prohibited foreigners from owning American-made ships, on the grounds they might be used for trading with Communist countries. He paid a $7 million fine.)

Evans frequently mentions names and concepts without the elementary journalistic expedient of identifying them in the first reference. For example, while Meyer is mentioned repeatedly in the first pages of the book, it is not until page 17 that we learn he was Onassis' aide-de-camp. Evans' habit of making a point and then supporting it with a quotation enclosed in parentheses is distracting. IN CONTRAST, L.J. Davis, a contributing editor of Harper's magazine, has written a masterful biography of Onassis -- a breezy, engrossing and credible tale that rivals Dallas for sheer glitz.

Without ever having met his subject, Davis convincingly portrays him as a "brilliant adventurer" who yearned to be accepted yet relished being an outsider, who sought to control the people around him but didn't know how and who, in the end, saw those he loved most turn against him.

His only son, Alexander, began taping his calls with his father to show his mistress how Onassis taunted him: "Onassis warbling 'Singin'in the Rain' for five minutes, interrupting himself with mocking questions ('How's the weather on your end?') to which no answer was expected, then resuming his song, only to conclude his performance with a screaming tirade, demanding and demanding and demanding, while Alexander responded with helpless grunts and monosyllables, his face a mask of defeat." But Alexander's death in a plane accident devastated Onassis with remorse. He considered, then rejected, cryogenics -- placing his son's body in a sub-zero crypt for eternity.

His only other child, Christina, had a dizzying number of affairs while going through a string of marriages. She couldn't sleep without a servant in the room, changed her underwear several times a day but never brushed her teeth, blew up to weigh 200 pounds and was admitted to a London hospital suffering from an overdose of sleeping pills. In the end, she inherited her father's empire and managed his ships reasonably well.

Finally there was Jackie, the rapacious spender, who married Onassis under the umbrella of a contract spelling out financial arrangements in the event of a divorce, who spent $5,000 for messengers for letters she didn't care to mail, who 23 times took his Olympic Airways planes and helicopters out of service for her personal use, who bought clothes and never wore them, and who was roundly hated by Christina and Onassis' lover, Maria Callas.

Davis is weak in describing just how this uneducated, once-poor Greek, who "perspired freely in all seasons," managed to make upwards of $1 billion. Other books plow these furrows more deeply: Frank Brady's Onassis, the London Sunday Times Insight Team's Aristotle Onassis, or even The Fabulous Onassis, by Onassis' man Friday, Christian Cafarakis. But no matter. A biography can never be complete, nor should it be.

Davis makes up for this lapse by bringing Onassis' personal life into sharp focus, demonstrating that what matters is not whether a biography is authorized or unauthorized but whether the writer has the ability to grasp his subject.

Ronald Kessler, a reporter on leave from The Washington Post, is the author of "The Richest Man in the World: The Story of Adnan Khashoggi."