One of the most complex topics in all of physics is string theory, and trying to explain that theory is quite a challenge. (I know, since I spent a year writing a book that does just that!) Here are a list of books which attempt to make the theory presentable to the average reader, with varying levels of success.
1. The Grand Design by Stephen Hawking and Leonard Mlodinow
Bantam press
In this bold book, the authors attempt to show that M-Theory can fully explain the existence of the universe without leaving any "gaps" into which needs be inserted a creating deity, or God. I believe that the excellent book falls a bit short of its highly-ambitious mark, though it's a great read nonetheless.
2. Many Worlds in One: The Search for Other Universes by Alex Vilenkin
Hill and Wang press
In this book, cosmologist Alex Vilenkin discusses how recent research into the origins of the universe, together with concepts from string theory, are making many physicists turn to the idea of a vast multiverse of possible universes. The theory of eternal inflation indicates that many "bubble universes" are continuously being created and destroyed, and implications from string theory indicate that there could be a vast number of possible energy configurations that a starting universe takes on upon creation. Vilenkin - and many others - are left with the idea that our universe is probably not the only universe, and that our physical laws are probably only a matter of chance.
3. The Trouble With Physics by Lee Smolin
Houghton Mifflin
In The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next, controversial physics gadfly Lee Smolin presents his theories that string theory is failing to meet the expectations that drove it to dominance as the theory of quantum gravity. There have been numerous attacks upon the details of Smolin's scientific arguments against string theory, but the far more compelling aspect of this book is his discussion in the latter third or so of the book that the scientific community is not receptive to new ideas. Even after intense reading on the subject, I'm not in a position to say whether Smolin is right or wrong on his science, but the discussion of scientific bias makes the book worth a read regardless.
4. Hiding in the Mirror by Lawrence Krauss
Viking Press
In another book in the skeptic camp with regards to string theory, Lawrence Krauss takes the opportunity in Hiding in the Mirror to explore the fascinating history of extra dimensions, both in science and literature. He points out how this concept has captured the imagination of people in the past, and how it continues to do so - mainly through string theory, which conjectures a number of extra dimensions. There are only a couple of chapters of this book devoted explicitly to string theory, but for those interested in a more general look at the ideas behind extra dimensions, this book makes the topic very accessible to readers of any background level.
5. The Trouble With Physics by Lee Smolin
Houghton Mifflin
Physics gadfly Lee Smolin, who likes to work on esoteric alternative theories of quantum gravity, has written a controversial book attacking string theory for an undeserved pre-eminence among the theoretical physics community. Though this book appears to be something of a centerpiece in the "string wars" across the physics blogosphere, the most compelling and important section is his indictment of sociology of the physics community. Specifically, he argues that it is not inclusive enough of women or minorities.
6. Not Even Wrong by Peter Woit
Basic Books
A physicist-turned-mathematician criticizes string theory on a number of levels, going into a bit more depth about the mathematical framework than Smolin's criticism. For those who want a glimpse into some of the mathematical concepts at the heart of quantum physics, while also seeing what opponents are saying about string theory, this is the book for you.
