Not a book but you might start with the free article by Edward Witten,
"What every physicist should know about string theory"
http://scitation.aip.org/content/aip/magazine/physicstoday/article/68/11/10.1063/PT.3.2980
From the article,
"Some of nature’s rhymes—the appearance of similar structures in different areas of physics—underlie the way that string theorypotentially unifies gravity with the other forces of nature and eliminates the ultraviolet divergences that plague quantum gravity.
String theory has, even among theoretical physicists, the reputation of being mathematically intimidating. But many of its essential elements can actually be described simply. This article aims to answer a few basic questions. How does string theory generalize standard quantum field theory? Why does string theory force us to unify general relativity with the other forces of nature, while standard quantum field theory makes it so difficult to incorporate general relativity? Why are there no ultraviolet divergences in string theory? And what happens to Albert Einstein’s conception of spacetime?..."
You may not understand it all but you will be exposed to buzz words that you will need to understand at some point.