The Subtle Knife( Series - His Dark Materials )

Lyra finds herself in a shimmering, haunted underworld--Cittagazze, where soul-eating Specters stalk the streets and wingbeats of distant angels sound against the sky. But she is not without allies: 12-year-old Will Parry, fleeing for his life after taking another's, has also stumbled into this strange new realm.
On a perilous journey from world to world, Lyra and Will discover an object of devastating power. And with every step, they move closer to an even greater threat--and the shattering truth of their own destiny.