She was being modest. The truth is that Scott used a great deal of Zelda's writing, credited to characters he modeled after her, in every book he completed in his abbreviated life. That Zelda was Scott's muse is hardly news, and it comes as no surprise that her frank sexuality, the wild abandon with which she flaunted her body at parties, gave color to his stories: More has been written about the Fitzgeralds, their antics and affairs, than they can possibly have known about themselves.

Yet, while others have certainly noted the spill of life into art, and even marked passages of Scott's books actually written by Zelda ("What grubworms women are to crawl on their bellies through colorless marriages"), Kendall Taylor's new biography of the couple, "Sometimes Madness Is Wisdom" (to be released in September) is the first to provide adequate groundwork for a thorough account of literary custody. Examining sources new and old to find just where within the Fitzgerald home plagiarism began, and at what madhouse it ended, Taylor attempts to make the case that "In effect Zelda was Scott's co-author."

Taylor's documentation is formidable, and were she simply out to argue that Scott could be a despicable creature, a liar and a cheat and a philandering drunk, we could shrug our assent and go back to Gatsby's house party or Dick and Nicole Diver's swath of Riviera beach. But the contention that, as literature, Scott's novels are in any meaningful degree a creation of Zelda is as insupportable as that the Mona Lisa be reattributed to the young wife of Francesco del Giocondo who sat, with that famous smile, as its model. Technically, Scott was a plagiarist. Artistically, that makes no difference.

Like their marriage, the Fitzgeralds' creative relationship went to extremes no couple could be expected to endure, not quite innocent from the start. Young Scott, an Army lieutenant stationed in Alabama awaiting orders to fight overseas, had always found it easy to interest girls by talking up his literary ambitions and asking them, "What sort of heroine would you like to be?" He quickly perceived, though, that to attract 17-year-old Zelda Sayre would demand more: Locals had to wait months for a date, and Army aviators vying with one another to get her attention regularly flew stunts over the Sayre family home risky enough to cause a collision.

So Scott, suited in a uniform of Brooks Brothers cut, not only boasted that he intended to be a famous author and had Francis Scott Key as an ancestor, but also suggested that the female lead in his novel-in-progress was a girl a lot like her. That was true -- albeit only because she resembled the young heiress who'd dumped him in Chicago. Still he intrigued her, enough to take him seriously, and try him out sexually, in spite of his poverty and her intention to marry wealth.

A tacit agreement was reached. As she expressed it to one of his Princeton classmates, "If Scott sells the book, I'll marry the man, because he is sweet." After that, she gave Scott all her support, sending him love letters full of spirited encouragement and quotable wit: A running account of night after night on the town with the heir to one or another Southern fortune.

Stung by jealousy, Scott used those letters, as well as material she let him copy from her diaries, to nuance the novel that would become "This Side of Paradise," a book he almost wholly rewrote to meet his image of her. But, while he flattered Zelda by showing her scenes in which she was depicted as could only be accomplished by a spectacularly talented writer in a state of hopeless infatuation, she cut off all sexual relations with him, and locked the engagement ring he'd offered her (borrowed from his mother) away in a box until he proved himself a literary success.