After You, by Julie Buxbaum.
How have I never heard of this author before? I love, love, love this book. Go right out to your nearest bookseller or library right now and get this book. What are you waiting for??!!
I don't know why this book hits me so hard, but I fell in love with Julie's writing style and her characters immediately.
I'll be checking out another of Julie's books, The Opposite of Love, as soon as I can get my hands on it.