Tomorrow, and Tomorrow, and Tomorrow by Gabrielle Zevin

On a bitter cold day, in the December of his junior year at Harvard, Sam Masur exits a subway car and sees Sadie Green.  She turns and a game begins: a legendary collaboration that will launch them into stardom.  Overnight the world is theirs.  They are brilliant, successful, and rich but these qualities won't protect them from their own creative ambitions or the betrayal of their hearts.  Spanning thirty years - it is a love story, but it is not one you have read before.