This book explains, in language understandable by (almost) anyone, two of the most important and least understood concepts in physics: quantum theory and the general theory of relativity.
"I think that I can safely say that nobody understands quantum mechanics", the great physicist Richard Feynman famously said. I'm sure he was right, and I certainly don't claim now to understand it, but at least after reading this book I feel for the first time that I have a basic grasp of some of the abstruse concepts involved, aided by the many incredible facts Chown sprinkles through the book, such as that the entire human race would fit in the volume of a sugar cube.
The second half of the book deals with Einstein's theory of relativity. The concepts here are far more familiar to me, but again Chown describes them in an elegant and accessible way, once again illustrating them with simple but memorable analogies and facts, such as that you age faster at the top of a building than the bottom.
Another reviewer complains that the book has no diagrams, something I noted with surprise when I first opened it. However, it is so clearly written that no diagrams are required - I certainly didn't ever feel I needed a diagram as I was reading it.
If you have an interest in these subjects (and, as they are central to everything in the universe, I think you should), then I thoroughly recommend this extremely readable book (even if it has a completely naff cover!).