Vacation houses are always such treasure troves of trashy novels. The summer before seventh grade, we rented a house in Chilmark, on Martha's Vineyard, and among the bosomy heroines and chiseled heroes that populated the bookshelf in my bedroom, I found Judy Bloom's Summer Sisters. To my surprise, it was not a continuation of Are You There God, It's Me Margaret, though its protagonist started off at the same age.
Summer Sisters is the story of an unlikely friendship between the wild and wealthy Caitlin and the reserved and impoverished Vix, and the summers the two spend at Caitlin's summer home on Martha's Vineyard. It's told mostly through Vix's eyes, and we watch the wax and wane of influences, watch Vix mature and Caitlin spiral through adolescence and into adulthood.
The book takes place over two decades, though the bulk of it takes place when the girls are teens. When I was 12 I found it incredibly raunchy; now it's just juicy, lighter-than-it-wants-to-be fun, and chock full of MV references. If you haven't read any of Bloom's adult fiction, and are in the mood for a true beach read, you may want to check this out.