Part ghost story, part problem novel, part droll view of the hazards of being adolescent and a writer's daughter to boot -- this never quite turns out to be much of any one thing. Nor does Adriana Prushtak -- A.D. for short -- who still totes a doll around at age thirteen, yet welcomes her first period with prideful calm (""Mother. . .I have entered your beautiful new world"") and copes marvelously with the mental disarray of Mrs. Cramer, her employer and sole adult companion during her Nantucket summer of babysitting. While Mrs. Cramer lapses into the impotent delusions of severe depression, A.D. tracks down and deals with the summer house's ""real"" ghost -- a disappearing ""hippie"" who's searching for the girl he was separated from back in the 19th century. All this, plus taking care of two year-old Stephanie and enjoying the shy companionship of her first boyfriend, makes for a busy summer. It's enough to distract other summer vacationers as well -- though that's about the highest level of involvement A.D. is likely to inspire.