Goldenhand by Garth Nix

Lirael is no longer a shy Second Assistant Librarian. She is the Abhorsen-in-Waiting, with dead creatures to battle and Free Magic entities to bind. She's also a Remembrancer, wielder of the Dark Mirror. Lirael lost one of her hands in the binding of Orannis, but now she has a new hand, one of gilded steel and Charter Magic. When Lirael finds Nicholas Sayre lying unconscious after being attacked by a hideous Free Magic creature, she uses her powers to save him. But Nicholas is deeply tainted with Free Magic. Fearing it will escape the Charter mark that seals it within his flesh and bone, Lirael seeks help for Nick at her childhood home, the Clayr's Glacier. Even as Lirael and Nick return to the Clayr, a young woman from the distant north braves the elements and many enemies in a desperate attempt to bring a message to Lirael from her long-dead mother, Arielle. Ferin brings a dire warning about the Witch With No Face. But who is the Witch, and what is she planning? Once more a great danger threatens the Old Kingdom, and it must be forestalled not only in the living world, but also in the cold, remorseless river of Death.