When You Reach Me - Rebecca Stead

By sixth grade, Miranda and her best friend, Sal, know how to navigate their New York City neighborhood. They know where it's safe to go, and they know who to avoid. Like the crazy guy on the corner.

But things start to unravel. Sal gets punched by a kid on the street for what seems like no reason, and he shuts Miranda out of his life. The apartment key that Miranda's mom keeps hidden for emergencies is stolen. And then a mysterious note arrives, scrawled on a tiny slip of paper. The notes keep coming, and Miranda slowly realizes that whoever is leaving them knows things no one should know. Each message brings her closer to believing that only she can prevent a tragic death. Until the final note makes her think she's too late.