When You Reach Me - Rebecca Stead

Miranda's life is starting to unravel. Her best friend, Sal, gets punched by a kid on the street for what seems like no reason, and he shuts Miranda out of his life. The key that Miranda's mum keeps hidden for emergencies is stolen. And then a mysterious note arrives:

'I am coming to save your friend's life, and my own.

I ask two favours. First, you must write me a letter.'

The notes keep coming, and Miranda slowly realises 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.