The Phantom of the Opera by Gaston Leroux
I really enjoyed this book; I loved the description of the labyrinthine opera house, and its cellars, and the "haunted" Box Five. The book portrays that atmosphere very well, and although Erki (the "Phantom") seems really nasty to begin with, you will eventually feel sorry for him, although the love story between Raoul and Christine was very compelling.
I loved all the myseriousness in this novel, particularly Christine being spirited away unexpectedly; one of the best books I have read recently.