Sabriel by Garth Nix

Sabriel is the daughter of the Mage Abhorsen. Ever since she was a tiny child, she has lived outside the Wall of the Old Kingdom--far away from the uncontrolled power of Free Magic, and away from the Dead who won't stay dead. But now, her father is missing and Sabriel is called upon to cross into the world to find him. Leaving the safety of the school she has known as home, Sabriel embarks upon a quest fraught with supernatural dangers, with companions she is unsure of--for nothing is as it seems within the boundary of the Old Kingdom. There, she confronts an evil that threatens much more than her life, and comes face to face with her hidden destiny.