Still one of the better books about the recent findings in String/M-Theory for the lay reader. Fortunately, the ridiculous abstract math is left in the endnotes for the brave. Say what you will about the validity of String Theory (or framework, or whatever), Brian Greene does a fantastic job at visualizing difficult and abstract concepts. He seems to be a veritable font of analogies that make brilliant sense and clarify ideas that only lines before made your eyes cross.
The book is nicely divided up, so even if you didn't want to learn anything about strings or Planck-sized physics, you could still get a nice summary of the major advancements in the field up to the standard model, including relativity, by reading the first half of the book and then calling it quits. If you're interested in something more, the second half is for you.
Along the way are some wonderful philosophical questions as they relate to the study of physics and Greene covers them with equal aplomb: What is the point of continued reductionism? How do such theories pertain to free will?
The ending chapters are nicely left open. Optimistic, without being assured, which is entirely, scientifically appropriate. The problem everyone seems to have is that String Theory is in a way divorced from typical scientific endeavor, led by experiment and observation. It is instead buried in complex mathematics that are supposed to reflect a universe that many of us find difficult to visualize - one of 10 spatial dimensions, and single dimensional strings (or multi-dimensional branes) vibrating at different energy levels to create the different subatomic elementary particles, which in turn assemble to create atoms, which create molecules, etc. etc. The point is, for all the predictive power of the equations (based on perturbation methodology - approximations no less) string theory has yet to provide a measurable quantitative fact. Sure it does provide a framework for melding relativity and quantum mechanics, but so could loop quantum gravity - without the extra dimensions! Proponents of course argue that we simply lack the refined tools necessary to find the evidence by measuring Planck level distances and energies, or that by proving supersymmetry and super-partner particles we could be on our way to confirming at least the usefulness of the theory.
All that being said, I like the idea of it. It's abstract and the discoveries of this "science" are on the blackboard in equations rather than in the laboratory, but the idea that human beings can delve into the fabric of the universe by understanding the fundamentals of mathematics is appealing to me. It's like conquering the world with sheer intellect and I look forward to the work of the LHC to see if indeed String theorists like Mr. Greene have succeeded, or failed spectacularly.