House of Leaves (Mark Z. Danielewski, 2000): It's perhaps too early to tell which books from the current millennium will be remembered as horror classics, but Danielewski's stunning debut is almost a sure bet. Fragmentary, hallucinatory and pervasively creepy, the book follows a family who discover that their house is much bigger on the inside that it is on the outside, full of strange, unaccountable passageways, and somehow alive. But that's just one layer of the narrative, with another story running right alongside the first. The physical novel, itself, is a maze, with pages that sometimes feature just one word, weird layouts, footnotes within footnotes (for works that don't exist) and other bizarre breakdowns of standard fictional text. House of Leaves truly messes with your mind.