Sabriel is set on a divided continent - one side resembles 1900 (or so) England, while the other is ruled by dark magic. Good world, plot, ideas, etc - however, as Mia mentioned, characterisation is slightly lacking (improved in the next book, though). Recommended, along with the two sequels.
The Keys to the Kingdom series is aimed for younger readers, with a slightly... wacky world, carried perhaps a bit too far and for too long.
Shade's Children is good post-apocalyptic sci-fi; it's been a while since I read it, though.
All good books, none fantastic though. Overall, Sabriel (and sequels) are probably the best.