2001: A Space Odyssey (The Book) - Arthur C. Clarke

I read this once as a child and once in my thirties. Both times it was like a punch in the face. But a happy punch in the face. Even knowing the outline of the story, there's a sense of tension and fear hidden in the details that I've never experienced in any other book. The philosophical themes are subtle and unpretentious, and tangle themselves effortlessly into the frighting sense of vastness and emptiness and time. Maybe the best proof there is that a book can be short and sweet and part of a low-brow genre and still blow most fine literature out of the water.