His Dark Materials 2: The Subtle Knife (Junior Edition)

Will is twelve, and he's just killed a man. Now he's on the run, determined to discover the truth about the father he's never known. Then he steps through a window into another world — where soul-eating Spectres haunt the streets of a city, and a strange girl called Lyra is searching for Dust. His mission and hers are mysteriously linked, and together they must find a powerful and secret object that people from many worlds would kill to possess.