by Philip Pullman 

"What is he? A friend, or an enemy?"

"He is a murderer."

Will has just killed a man. He's on the run.

His escape will take him far beyond his own world, to the eerie
disquiet of a deserted city, and to a girl, Lyra. Her fate is strangely
linked to his own, and together they must find the most
powerful weapon in all the worlds...

His Dark Materials 2 The Subtle Knife