The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory is a book by Brian Greene published in 1999, which introduces string and superstring theory, and provides a comprehensive though non-technical assessment of the theory and some of its shortcomings. In 2000, it won the Royal Society Prize for Science Books and was a finalist for the Pulitzer Prize for General Non-Fiction. A new edition was released in 2003, with an updated preface.
It's been awhile since I read it, but I remember it having a definitely stringy bias (it's even in the title), so not the first place to go for a neutral over-view of approaches to quantum gravity if you are looking for that sort of thing.
