A popular book about string theory that claims to be readable even by Peter Woit has been finally written by George Musser. It's called The complete idiot's guide to string theory. Click the amazon.com links on the left to buy it.
Amanda Gefter: From the vibrations of tiny strings comes an orchestral crescendo: atoms,molecules, people, planets, stars, galaxies, the universe, the multiverse and ultimately - or so the dream goes - everything. George Musser presents the multidimensional world of string theory in this fantastic primer on all things physics, including the problem of time, the holographic principle and alternatives to strings. The book is not only for beginners, but its lucid explanations ensure that even an idiot will emerge with an ear for the strange music of reality.