Hint: Many GU books are titled after the protagonist. If a young adult fantasy has a female name for its title, I always check the plot – often as not, it fits. Sabriel by Garth Nix is no exception.
At the age of 18, about to graduate school, Sabriel has lived most of her life in Ancelstierre, a country on the other side of a wall which separates it from a magical land called the Old Kingdom. Her father is a famous and special sort of necromancer, called the Abhorsen, who banishes dead spirits that have risen back to the afterlife – he visits only rarely and teaches her too little of his work, though she learns some magic at her school. One day she receives a message from her father – he is trapped behind the gates of death, and dangerous spirits are abroad. Sabriel must journey alone to the Old Kingdom to find his body, try to rescue his spirit, and then hopefully repair some of the damage that has been done in his absence.
After defeating or delaying a few malicious creatures at the outset of her journey, she reaches her father’s home and acquires a companion – a spirit in the form of a cat named Mogget. He and her father’s other spirit-helpers assist her in defending the home long enough for her to escape. As Sabriel and Mogget continue on their quest towards the royal city in the north, she learns the nature of her adversary (a mad sorcerer) and his terrible undead minions, as well as the fact that she is now herself the new Abhorsen, and expected to do the work of that office, which she has not been well prepared for. She also gains another companion when she frees a man from a 200-year-old spell.
Together they reach the royal city and the exact place where the adversary both began his assault on the whole kingdom, and entrapped her father. She frees her father’s spirit, but is unable to save his life. Meanwhile, the adversary is only delayed a little by her efforts. Sabriel and her companions must return to Ancelstierre and find the adversary’s mortal body and destroy it. But of course, he is hot on their heels and may bring an apocalypse in his wake.
In the final face-off, one companion is injured and the other has his own agenda, so Sabriel must find a way to defeat the adversary herself. She finds a clever way to subdue him, although it may not be the final end of him.