Increase Intelligence?

In the past few weeks I've become obsessed with increasing my overall intelligence, and everything I've read have all had one thing in common, read more books. Well, I like to read but the problem it's hard for me to find good, challenging books. I've just read Angels & Demons and The Da Vinci Code and really liked those books. I've also started A Brief History of Time by Stephen Hawking, which is really interesting. What I want to know is, are there any books out there you recommend, and are there other ways to increase my overall intelligence, any help will be appreciated because I will not stop thinking about this for a couple weeks, at least.