A SEASON IN PURGATORY By Dominick Dunne (Bantam Press, $255) IF you can remember what you were doing when you heard John Kennedy had been shot dead, you will probably not appreciate this book to the fullest. A large distaste for political icons, particularly of the dynastic kind, is essential here. Also, true belief in the dictum that power corrupts. Dominick Dunne makes it quite clear that the rich, powerful Irish Catholic family from Connecticut which crowds his new novel is modelled on the Kennedys. ''Same dimpled chin. All those big white teeth. A head that will never go bald,'' Mr Dunne writes of one those unmistakable Bradleys. With minor variations, all the key members are present - or absent, as in case of Agnes, the retarded daughter who languishes in an asylum and Kev who was killed in Vietnam. He was to be the great hope, all the way to the Oval Office. Being groomed in his place is youngest son Constant, the most spectacular heir apparent in Catholic America. But Constant also has a dark side. He roughs up girls - nice girls - and one night, he goes too far. The inspiration for Constant came from two sources: the rape trial of Willie Kennedy Smith, which appalled Mr Dunne, and the unsolved case of a 15-year-old girl, murdered 18 years ago in Greenwich, Connecticut. In a recent article for The Times, writer Valerie Grove noted that ''the chief suspect has always been a nephew of Ethel Kennedy, wife of the assassinated Bobby'' and quoted Mr Dunne as saying that since the publication of his book, vital new evidence had come to light which he planned to tell to the police. This may spell further notoriety for the Kennedys. In the meantime, Mr Dunne can rest assured that he has turned on the spotlight to full glare. He writes with inside knowledge. Like the Kennedys whom he has known since boyhood, the author grew up in a privileged, Catholic, New England family. Like Constant's best friend, Harrison Burns, who relates much of the story, he has won fame for chronicling crimes committed by those who ''get away with things, mostly'', because of their enormous power, wealth and often, charisma. Mr Dunne's alter ego finds himself helplessly drawn into a web of deceit and conspiracy. The price is his season in purgatory: 17 years which see Burns grow from an introverted youth into a personable best-selling author, though no amount of success can lessen the torment of knowing how Winifred Utley met her death. Far uglier than the crime and even the nature of the criminal, is the cover-up. There is no horror at the mortal sin which has been committed, no compassion for the anguished parents; only cold-blooded determination to protect Constant from a potential disaster. Mr Dunne captures the New England snobbery to perfection. He also gives tacit approval to it with his devastating portrait of the senior Bradleys. There is pious Grace in her Paris designer clothes, praying mightily to be made a papal countess and religiously checking that all in her mock-Tudor pile is placed exactly as the interior decorator said it should be. And there is Gerald, model family man and rampant philanderer, pillar of the church and the Mafia. Ironically, it is his favourite child who can see through the monstrous sham, but even Kitt - so like Constant, but with a heart and soul - cannot escape the inevitable. Faced with death or dishonour, she knows there is only one option for a Bradley.