Late one night last April, Fidel Castro rose to address an outdoor rally of the Union of Communist Youth in Havana. Party officials prompted the throng to greet him with a playful nationalist chant: "El que no brinque es Yanqui" - "Anyone not jumping is a Yankee." Thousands of young Cubans bobbed up and down. Only a handful of people didn't follow suit: my fellow foreign journalists and I (we were Yankees) - and Fidel Castro. Glowering through his whitening beard, Fidel merely moved his hands up and down. The strange, stiff motion was intended, I suppose, to symbolize jumping.

Of course, Fidel Castro is the one person in Cuba exempt from compulsory public displays of loyalty; he can choose to jump or not without fear of being labeled a Yankee. But his odd behavior that night was also characteristic of the extent to which Castro, after 32 years in power, has gradually excised spontaneity and humor from his public persona.Nowadays, everything that once seemed fresh and fun about Fidel has become solemn, ritualized. His jaunty combat fatigues have evolved into a political-military costume. His personal life is a state secret subject to painstaking official historical revision.

Indeed, the more desperate conditions have gotten in Cuba, the more energy Fidel has devoted to his aura of revolutionary gravitas. In the last year, as communist Cuba's isolation has deepened, along with its poverty and official corruption, it has sometimes seemed that the essential function of the Cuban state is to deny that any of the island's problems could be the fault of the man in charge.

Georgie Anne Geyer is intent on debunking these official myths. She bills her book as an "unprecedented" tale, an "untold story" that will blow the cover off what she considers to be Fidel's unbroken record of political and (especially) private transgression. Her account is based on dozens of secondary sources and an extraordinary number of interviews with people who knew Fidel in the difficult years before and just after he seized power in 1959 at the age of 31. (One nugget: Richard Nixon, who met Fidel in Washington in 1959, calls him "someone I'd like to have on our side.")

Much of what she has dug up will, no doubt, make Castro wince. It was widely known, or believed, in his rural Cuban home town that Fidel was illegitimate. He raged against his father, Angel Castro, even while collecting a $100 per month stipend from the old man - then showed no grief at all upon hearing of Angel's death. He abandoned his upper-class wife, Mirta Diaz-Balart, to pursue politics, then attempted to kidnap their only son from her and give him to a Mexican family Mirta barely knew. Since then, Geyer reports, Fidel has used a variety of women as playthings.

Geyer musters this information to show that the origin of Castro's appetite for power and conflict does not lie in communist ideology, to which he committed himself belatedly in exchange for Soviet support against the United States. Rather, she argues, his political excesses stem from his own life.

Fair enough. But the question is, just how new is Geyer's "unprecedented" thesis? Similar psychobiographical arguments have appeared already, notably in "Castro, Cuba, and the World," by Edward Gonzales and David Ronfeldt of the Rand Corp., and in "Insider," the memoir of Cuban defector Jose Luis Llovio Menendez. (The former book is cited in Geyer's bibliography; the latter is not, although she interviewed its author.)

So bent is Geyer on finding Castro's hand behind all the upheaval in the Americas since 1959 that she ventures into sweeping assertions that are poorly sourced, or not sourced at all. How does she know, for example, that Castro "personally oversaw" the military operations of Nicaragua's Sandinistas during their 1979 battle for power, issuing them "precise orders"? How does she know that in 1980 Castro planned to move the Americas Department of Cuban Intelligence to Michael Manley's Jamaica? The problem is compounded by Geyer's system of citation. She simply lists the books or interviews used for each chapter, without mentioning which facts came from which source.

The text is marred, too, by factual and spelling glitches. Managua, Nicaragua, was leveled by an earthquake in 1972, not 1970; Cuba's former Interior Minister spelled his name Abrahantes, not Abrahante; El Salvador's guerrillas are commonly known as the FMLN, not the FMNLF. Geyer lapses into ungainly writing: More than once she uses the adjective "behemothic," and at one point ascribes to Castro "a fury so consuming it would have burned the calluses off the feet of Lucifer himself."

For all its errors of omission and commission, "Guerrilla Prince" is a worthwhile corrective to many of the cliches that cloud the discussion of this complex, magnetic man.