Nonfiction: As someone who leans toward biology, I've found Richard Dawkins to be consistently fun and thought-provoking. For explaining hard-to-understand concepts clearly, Stephen Hawking's "Brief History of Time" and "The Universe in a Nutshell" are great. For general awesome, "Surely you're Joking, Mr. Feynman!" is both educational and funny.

Fiction: Neal Stephenson is a genius at world-building. I've read "The Diamond Age," "Snow Crash," and "Anathem," and they've all been hits.