If the OP admits to being not much of a reader and is interested in Harry Potter and Dan Brown (gasp) I don't think a 645,000 word novel with a 30 page monologue is really a good suggestion. I read fairly often and Atlas Shrugged was the hardest book I've ever read. Ayn Rand tends to repeat herself over and over to the point of annoyance.
As for the original suggestion, try Tom Clancy or Michael Crichton, both easy reading and fun. You could spend months reading the Jack Ryan series alone, just do it in order. Crichton just has alot of good novels, State of Fear is good and I really liked The Great Train Robbery.
An even easier fiction series to read is the Lord Ramage series by Dudley Pope. Think of these as the kid version of Master and Commander, everything is explained so you're not trying to decipher nautical slang and read a novel at the same time like O'Brian.
Some good non-fiction:
The Elegant Universe by Brian Greene
It's a physics book, I'm not gonna lie, but it's all about how the universe is put together and whatnot. Probably one of the best explanations of relativity I've ever seen.
Dragons of Eden by Carl Sagan
It's about the evolution of human intelligence. Not hard to read or understand, just lots of interesting things about your own brain.
Freakonomics and The World is Flat (I forget the authors) are both good books about the world around us. Impress your friends with your knowledge of world affairs type stuff.