Death from the Skies!: the Science Behind the End of the World, by Phil Plait
Phil Plait has a really enjoyable writing style and is very good at explaining difficult scientific concepts clearly. In this book, he explores several different possible ways for the world to end, each less probable than the last (or at least, very very far in the future). It's a great book for anyone interested in astronomy; the explanations of the Sun and supernovae are the best I've read. I plan to require my kids to read it as part of logic-stage astronomy.