Sabriel (Abhorsen, #1)
by Garth Nix
Reread it for the first time in 8 or 10 years because Sam really likes it and (nerdery of nerderies) we're currently playing a D&D campaign based on it. (I'm teenage Chrestomanci, because... um.)
Getting back to the book, I was surprised by how little I remembered and how great the atmosphere is. Always liked Lirael the best (must reread it now), and never got past a few chapters of Abhorsen, so that comes next.