Abhorsen Trilogy

Before she went to join Sam and Mogget downstairs, Lirael paused for a moment to look at herself in the tall silver mirror that hung on the wall of her room. The image that faced her bore little resemblance to the Second Assistant Librarian of the Clayr. She saw a warlike and grim young woman, dark hair bound back with a silver cord rather than hanging free to disguise her face.

Ever since she was a tiny child, Sabriel has lived outside the walls of the Old Kingdom, away from the random power of Free Magic, and away from the Dead who won't stay dead. But now her father, the Mage Abhorsen, is missing, and to find him Sabriel must cross back into that world.