Old Kingdom Series
The Old Kingdom series (also known as the Abhorsen Trilogy in the USA) is a collection of young adult fantasy books written by Garth Nix featuring the novels Sabriel, Lirael and Abhorsen. There has also been a collection of short stories set in the Old Kingdom called "Across the Wall"
The Old Kingdom is a land where magic is commonplace, Sabriel and Lirael are training to become a type of reverse necromancer and live outside the old Kingdom in a world very much like our own (called Ancelstierre).
Sabriel is the daughter of the Mage Abhorsen (another reverse necromancer) whose role is guiding the disturbed dead back to rest. When Abhorsen goes missing, Sabriel must leave the security and safety of her school to travel into the Old Kingdom to find him.