The Subtle Knife by Philip Pullman

She had asked: What is he? A friend or an enemy?

The alethiometer answered: He is a murderer.

When she saw the answer, she relaxed at once.

Lyra finds herself in a shimmering, haunted otherworld–Cittagazze, where soul-eating Spectres stalk the streets and wingbeats of distant angels sound against the sky.

But she is not without allies: twelve-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 uncover a deadly secret: an object of extraordinary and devastating power.

And with every step, they move closer to an even greater threat–and the shattering truth of their own destiny.