The Elegant Universe, by Brian Greene, is a guide to physics in general and superstring theory in specific.
Greene starts out by explaining, in simple terms, the basic ideas behind the theories of relativity and quantum physics, and how they’re increasingly coming into conflict.
Relativity dealing with massive things moving quickly, and quantum physics dealing with tiny things, the theories have managed to coexist for a while, but the discovery of singularities like black holes, which are both massive and tiny, need something to tie it all together without actually getting infinities anywhere (be it infinitely small sizes, or infinite densities or temperatures, or what).
Clearly, string theory is the solution, and the second half of the book is devoted to explaining why this is the case.
Keeping in mind that string theory hasn’t made any predictions yet that can be tested with current technology other than ones that can be explained by other, older theories as well (which Greene openly acknowledges), he makes a pretty good case.
Without going into the underlying mathematics, he manages to explain how the various particles and forces we observe (including gravity and the as yet undetected graviton) flow naturally from string theory, and how it seems to accomodate supersymmetry, which is just pretty, and how the theory is really too elegant not to be true (which, I’ll grant, isn’t a valid argument on its own).
It’s a very interesting book, both for the physics (even if you dislike string theory, the bit about relativity and quantum physics is good enough in its own right) and for the history lessons. The way string physicists approach mathematics is, of course, obnoxious, but even that isn’t too bothersome.
Definitely worth reading, even—or especially—for those with no background in physics whatsoever.