Bardos, the Necronomicon is a wonderful idea *smile*.
Well, of all books, I'd choose a philosophy book, and probably one of the most difficult books, so it would fascinate me very long (I think, something by Nietzsche, but this may be because at the moment I'm in kinda Nietzsche-Period - I just heard more of his theory at school and it amazes me)
If it has to be a fantasy book, I think LotR would be appropriate...