The Picture of Dorian Gray by Oscar Wilde - extremely clever writing
Hamlet by William Shakespeare - easily his best
The Hitchhiker's Guide to the Galaxy by Douglas Adams - hands down the funniest book I've read
Anything from the "... and Philosophy" series. These are collections of essays, articles and papers written about elements of pop culture. Books in the series include Seinfeld, Star Wars, Harry Potter, South Park .... with my favorite being The Simpsons and Philosophy: The D'oh of Homer. They are intellectual essays written by philosophy professors. They're not dumbed down in any way, they're entertaining and they'll really make you think.