I'll throw Mark Twain in here. Tom Sawyer is all fun and games. On the surface Huckleberry Finn is the same, but there is so much more there. The Mysterious Stranger is as dark as a work can get, especially considering the time it was written.

Vonnegut is a great author who is crossing generations. I remember picking up my Dad's copy of Cats Cradle, then seeking out everything else I could find. Years later my younger brothers brought home Slaughterhouse Five as homework. They got hooked and I turned them onto my books. I recently watched my oldest son go through the same thing. An author who can cross those age gaps and still seem relevant has a gift.