String theory is thought by many to be the best possibility for a unified theory of physics. In The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory
Brian Greene gives a non-technical overview of this theory. The book starts with an introduction to various parts of modern physics, such as quantum theory, special and general relativity as well as particle physics. Greene then explains the quest for unification, and why the unification of gravity and quantum theory has presented such problems. He goes on to introduce string theory as a proposed solution to these problems, and shows how it incorporates supersymmetry (and so is sometimes known as superstring theory) as well as explaining where all the extra dimensions come in.
In the later part of the book Greene discusses the cosmological implications of string theory, looking at their behaviour in the early universe as well as how they come into the theory of black holes. There's also a look at M-theory, which brings together different versions of string theory. The book concludes with a look at the future of string theory (although it was written before most of the current controversy about the subject started).
Some of the concepts involved in string theory can be hard to grasp, but Greene is a skilled writer and for those of you wanting an introduction to the theory, I would say that this book is probably your best choice.