Sabriel is the first book in The Old Kingdom Series by Garth Nix.
Plot:
A large wall separates the Old Kingdom from Ancelstierre. But it’s more than just a geographical separation: there is magic in the Olde Kingdom, Ancelstierre on the other hand relies on technology. Sabriel is from the Old Kingdom but goes to school just south of the wall in Ancelstierre because her father wanted to make sure she was safe. In the Old Kingdom she wouldn’t have been since her father is the Abhorsen: a necromancer charged with making sure that the dead stay dead. But then the Abhorsen goes missing and it is down to Sabriel to find him before the dead overrun both countries.
I had been meaning to re-read the entire series for a while, but when it was announced that a prequel was about to be published, I had to re-read it immediately. Sometimes I just make really good decisions because Sabriel is still as awesome as I remember it to be.