Heart of Darkness by Joseph Conrad
Heart of Darkness is not a walk in the park. The language is dense. The subject matter is bleak. It deals with themes both complex and horrifying, and it does so with symbolism and allegory. Though English was not his first language (it was actually his third), Joseph Conrad somehow managed to wield a mastery of English prose with this disturbing yet weirdly hypnotic exploration of humanity's darkest recesses.