If there's ever been a writer whose covers should not be used to judge their books, it's Philip K. Dick. His works are usually classified as a science fiction, but to me, he's a cyberpunk physicist, a time-traveling gnostic, and a prescient speed freak. He's also responsible for a slew of Hollywood films like Bladerunner and Total Recall, and the list goes on. I'm a fan of his, and that, of course, makes me a Dickhead.
So, I know all you other big Dickheads out there will be ecstatic to know the AV Club has a pointer to the Internet Wayback Machine's collection of over 600 Philip K. Dick book covers! It's a treasury of artists vainly trying to capture insanity, delusions, and false realities.
My favorite ones are those where the artist clearly has a minimal idea what the book is about.