"E=mc^2 - A Biography of The World's Most Famous Equation " by David Bodanis.
I just finished this -- it may not be the best book ever, but it's a great book if you're not a physicist or mathematician, and you want to get your brain at least partially "wrapped around" Einstein's Special Theory of Relativity. There is also great "background" information. (Germany came very close to having the atomic bomb, but I shall spoil no more.)