The best popular book on the topic of String Theory has got to be Brian Greene's "The Elegant Universe." After reading that book, I found that I finally understood quite a bit about what this theory really means.
String Theory, now called Superstring Theory due to its inclusion of supersymmetry, is gradually unifying its varieties of theories on strings into one large theory called "M-Theory". M-Theory uses an 11 dimensional universe, with three extended spatial dimensions and one time dimension. The rest of the dimensions are curled up in a Calibi-Yau shape, which I can't even begin to explain.