When You Reach Me by Rebecca Stead

It's funny how one seemingly insignificant event can change your life. For Miranda, that event involves her best friend Sal and an unexpected fist. When Sal gets attacked by the local bully, something drastic changes between them. Suddenly it's no longer Miranda and Sal against the world. Instead, Sal shuts her out, leaving Miranda alone and confused in their tough neighborhood. Soon after, Miranda receives a mysterious warning from what seems to be from the future; Sal is in danger. At the stranger's insistence and with his help in the form of four letters, Miranda hopes to save Sal's life as well as their friendship.