The Last Summer (of You and Me) by Ann Brashares.
This is Ann Brashares' first novel aimed at adults. (Although I read the Travelling Pants books and enjoyed them very much, they were classified as YA).
It's about Alice and her older sister Riley, and Riley's best friend Paul. They spent summers together on Fire Island, New York while they were growing up. When the book begins, Alice, bnow in her twenties, is waiting at the ferry depot for Paul, who hasn't been to the island for a few years. She is a lot younger than Riley and Paul, and adores both of them.
Although Riley and Paul fall into their old friendship, Alice is not quite sure where she fits in Paul's world anymore. Although written in the third-person, much of the story is from Alice's point of view, as she tries to figure out who she is, especially in relation to her sister and Paul.
Like the Pants series, this book is very much about relationships, personal growth, and life changes. The story is absorbing, and the characters very easy to empathize with.