Anything by Stephen Hawking.

Anything by Carl Sagan.

Actually, I’m not a fan of Stephen Hawking’s writing. Carl Sagan is good, but it’s a bit dated.

Brian Greene The Elegant Universe is really good until you get to the last third and he goes off on string theory. Fabric of the Cosmos is also good. Timothy Ferris The Whole Shebang is very good, but also a bit dated—it’s 20 years old now, but probably the best written popular science book on cosmology I’ve ever read.

I’d definitely say Brian Greene, but I’m not a String Theory fan anymore, but being informed about that theory is certainly important. On the other side of the theory spectrum, I’d read Roger Penrose (who has cooperated with Stephen Hawking on a number of books and subjects) and Alan H Guth http://www.amazon.com/The-Inflationary-Universe-Alan-Guth/dp/0201328402
and http://www.amazon.com/First-Three-Minutes-Modern-Universe/dp/0465024378/ref=pd_bxgy_14_2?ie=UTF8&refRID=0WRVNG57EW6GREA0SWT5

Wrinkles in Time by George Smoot

Great suggestions everyone. Thanks!