I had a gentleman of about 10 or 11 years old come to me during our Sunday hours at our Southeast Branch with a serious question: "Can you tell me where the books about black holes are?" It turns out, he knew more about black holes than any children's book would show him, but he still wanted pictures of black holes. We ended up finding a couple in more general books in our 500s section--the sciences. He blew me away with his knowledge: that black holes are portals to the other side of the galaxy, that there is a black hole in the center of our galaxy, that there are different types of matter but he personally doesn't believe in the dark matter theory...
For this person, the books about science opened up a realm of possibilities. Sure he could have chosen the more useful books in the 500s on geometry, or different types of insects, or how the weather works. But he chose the more fantastical side of science last weekend, and for that I thank him for the best conversation I've ever had about science. In his honor, I now present to you my favorite books on the subject of black holes and the universe:
The Illustrated A Brief History of Time
The Elegant Universe
String theory is as complicated as Stephen Hawking's work, but made even harder because its hard to understand string theory without the math. In any case, Greene makes it interesting and not too boring.