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.