Does anyone else remember Diana Wynne Jones's Crestomanci series? I loved her books as a kid -- and still do!
Nicole Luiken introduces a different take on mirror worlds, and I was instantly enthralled. Deftly juggling two storylines and worlds, the book is engrossing.
I don't read a whole lot of YA, but evidently I'm missing out! Great mix of fantasy and paranormal romance with engaging heroines, swoon-worthy heroes, and a fascinating villain -- a great villain can make a book!