The Last Summer by Ann Brashares

Riley and Alice, two sisters now in their twenties, and as fiercely different as they are loyal, have spent every summer at their parents’ modest beach house on New York’s Fire Island. Each year, they return to the house and community they have known since they were children—and to Paul, the boy next door. But this summer marks a season of change: budding love and sexual interest, an illness, and a deep secret force all three to confront the increasing complexities of their lives and friendships.