One of the first series I truly became obsessed with. And this obsession has continued long into adulthood.

Sabriel by Garth Nix (The Old Kingdom #1)
Lirael by Garth Nix (The Old Kingdom #2)
Abhorsen by Garth Nix (The Old Kingdom #3)
To Hold the Bridge by Garth Nix (The Old Kingdom #3.7)