Sabriel (Young Adult) Sabriel by Garth Nix - this is the first book of the Abhorsen trilogy, and after reading it I was left wondering why I'd put off getting around to it for so long.
The eponymous Sabriel is our protagonist, brought up in a sheltered school south of the Wall that separates the Old Kingdom, where the Dead walk and magic works, from Ancelstierre which is all electricity and rationality. Fortunately for Sabriel, she has been taught by her father, the Abhorsen, and a farsighted teacher at her school how to deal with the Dead - what she doesn't realise is that this is all preparation for eventually taking on the role of Abhorsen when her father dies.
Her settled and quiet life is thrown into disarray by a message from her father, whose body has been separated from its spirit and who is now trapped in Death. The role of the Abhorsen is to ensure that the Dead who walk among the living are sent back where they belong, but it also works the other way, giving Sabriel the power to rescue her father and set things right.
This is something of a traditional quest story, where Sabriel picks up companions along the way, all of whom will help (to some extent) with her task as well as helping her to understand her heritage and responsibilities. If it has a flaw, perhaps Sabriel accepts everything she is asked to do a little too blithely, but it's an interesting story set in a universe which has been well-built to accommodate it. The trilogy continues with Lirael and concludes with Abhorsen.