Kathy Boudin has served 17 years in prison for her role in a 1981 Brink's robbery that left a guard and two police officers dead and nine children fatherless. She received a sentence of 20 years to life, but in most cases an inmate with her record of model behavior could expect parole. Few people were surprised, though, when Boudin was turned down for parole at a widely publicized hearing in August. Prison officials cited the extraordinary violence of the crime. But looming over the case was Boudin's membership in the Weathermen, a spinoff of Students for a Democratic Society. A group of affluent white kids, they played at being revolutionaries during the 1970's and took credit for bombing two dozen public buildings, including the Pentagon. The group was defunct by 1981, when Boudin joined a band of thugs associated with the Black Liberation Army -- an offshoot of the Black Panther Party -- which executed the Brink's truck robbery and, to judge from bomb paraphernalia and plans found after the robbery, seemed ready to blow up several New York City police stations. The robbery made clear the extent to which the student protest movement of the 1960's had deteriorated into naked criminality.  Boudin, at 58, is confronting the possibility of spending the rest of her middle age in prison. But her former comrades Bill Ayers and his wife, Bernardine Dohrn, members of the Weathermen high command -- once known as the Weatherbureau -- who led the group through its most violent period in the 1970's, have served no significant jail time. Both of them teach at name-brand universities and are headed for cozy retirements like those of the bourgeois parents they so despised during their Weathermen days. Ayers has further cushioned his future by writing a maddeningly evasive memoir, ''Fugitive Days'' -- one of those books that tell by not telling.  The jacket copy is the kind of agitprop that could have been written by the young Ayers himself. In it we learn that ''Bill Ayers was born into privilege,'' and we are given to understand throughout ''Fugitive Days'' that privilege is a crime, if not a badge of shame. How did young Billy Ayers grow into a bomber? To hear him tell it, he saw too many war movies and played with fireworks supplied by his grandfather, who taught him the ''proper awe and reverence'' for an explosion that rattled the farmhouse windows. When he describes his family's comfortable life in suburban Chicago, the kitchen appliances, the color television and the maid are rendered into a bill of particulars against the Ayers family in particular and white suburbanites in general. Bill resents the father's good job, and the fact that he was once considered as a potential Secretary of the Army. He resents his mother's chirpy optimism and portrays her as blind to the social evils that will require his revolutionary attentions down the road. The class guilt drums on and on. It finds a predictable outlet when Ayers picks up Karl Marx in prep school, attracted to him because ''everyone else considered him beneath contempt, choosing the obvious -- Plato and Kant and David Hume.'' Ayers deems it crucial that Marx's children referred to him as the ''Old Moor'' -- which, I must admit, is a new one on me.  The Weathermen's other great theme was blackness. They fetishized it. Not just any blackness, mind you, but poor, angry blackness that tended toward violence and criminality.  This theme plays big as Ayers joins S.D.S. in the 1960's and sets up house in a Cleveland ghetto. There young radicals sing Negro spirituals and eat chitterlings with the natives, whose intelligence and industry they find surprising. Ayers has the obligatory 60's erotic encounter with a black woman. By the end of his stay in Cleveland, he is mesmerized by the incendiary violence of the 1966 race riots: ''By that time,'' he writes, ''I . . . thought I was black.'' When the Weathermen move underground, he likens the group to ''black Americans who must know everything about the dominant culture while remaining . . . invisible to that culture.'' When the group blows up a building, the act is cast as revenge for the power structure's ruthless attacks on the ''black struggle.'' This affinity, by the way, is what landed Kathy Boudin -- Bryn Mawr graduate and famous lawyer's daughter -- among the criminals who robbed that Brink's truck.  ''Fugitive Days'' contains a great many obfuscations. Chief among them is the author's reluctance to mention Boudin. The two had a longstanding acquaintance that probably began in Cleveland during their S.D.S. days. They would certainly have known each other by 1969, when the Weathermen split off from S.D.S. in pursuit of a more perfect radicalism. Years later, after Boudin went to jail for the armored truck robbery, Ayers and Dohrn took custody of her young son, Chesa, who was not yet 2 years old. The absence of Boudin in the book is peculiar, especially since she became famous in the event that forms the emotional center of ''Fugitive Days,'' the explosion that leveled the Greenwich Village town house the Weathermen were using as a bomb factory in 1970. Boudin survived the explosion, walking naked from the wreckage and onto the F.B.I.'s most wanted list. But the blast killed three people, including Ayers's lover, Diana Oughton, who was later identified from a fragment of finger.
FUGITIVE DAYS   A Memoir.   By Bill Ayers.   293 pp. Boston:   Beacon Press. $24.