Frankenstein by Mary Shelley
Well, that was a pleasant surprise. A much better story than any of the movie versions and surprisingly readable considering the early 19th century prose style.
The creature is a much more sympathetic character than expected - the reader can easily see how he 'turned to the dark side', considering his treatment.
I also found Mary Shelley to be a much better writer than I gave her credit for - I had assumed she just dabbled in writing in an amateur way, at the urging of her old man, Percy, and that he would be the talent in the family, but Mary's writing really flows and makes for a decent page-turner.