Saint X
A shaggy novel, with lots of incident and detail included for the joy of incident and detail. A New York family goes to the Caribbean island of Saint X for a luxurious vacation, father and mother and two girls. Princeton freshman Alison is hot stuff and knows it, little sister Claire is a little strange and knows it, too. On the final night of the vacation, Alison sneaks off and then vanishes; her body is eventually discovered on nearby, uninhabited Faraway Cay. Everything is changed.
This is formally a murder mystery and the little sister, who changes her name to Emily, serves as its detective. Nobody here is reliable, least of all Claire/Emily. You can’t rely on the police. And in the end, you cannot rely entirely on the author.