The Subtle Knife — Philip Pullman

Having slipped through a newly formed astral portal, the intrepid Lyra finds herself in the beautiful, haunted world of Cittagazze--a city where soul-eating Specters stalk the streets and the wingbeats of distant angels sound against the sky. But she is not without allies. For young Will Parry, in search of his father, has also stumbled into this strange new realm via a magic gateway.

Together the enlightened pair forge ahead on a perilous journey between worlds teeming with witches, angels, and sorcery--and uncover a deadly secret: an object of extraordinary and devastating power. But with ever, step, they move closer to an even greater threat--and the shattering truth of their own destiny ...