Beginning in the 16th Century, physics started to change from a purely scholastic mode of inquiry, in which questions were answered by argument from first principles and ancient authority, into a scientific one, in which observation and mathematical law predominated. With the introduction of Newton's work and his (and Leibniz') invention of the calculus, physics became a modern science, in which mathematics played a key role not only in testing theories, but in predicting phenomena as well. Even so, it was still possible for the non-scientist to understand much of the work of physicists, as it still dealt (for the most part) with laws and phenomena that could be observed, experienced, or at least imagined with the average person.
With the advent of relativity and quantum mechanics in the early 20th century, this all changed. Special Relativity dealt with velocities far beyond that which any human could ever experience. General Relativity dealt with interactions on a cosmic scale. And quantum mechanics dealt with scales far smaller than that which could be experienced or observed- even by physicists. What these new disciplines shared was that they they could only be truly understood by someone conversant with the mathematics involved. Although mass-induced curvature of space (for example) is commonly explained by analogy to a weight on a rubber sheet; that's at best, a weak metaphor. A ball bearing rolling on a rubber sheet is still being pulled down by gravity; it is not tracing a path in curved space that minimizes action.
As modern theories physics have become more complex, more purely mathematical, and further removed from the experience of the perceivable world, the books that attempt to explain things like inflationary cosmologies, string theory and supersymmetry have become increasingly less satisfying. And that brings us to the central failure of almost every popular book on modern physics I've ever read- the inability to actually explain the why and how. After reading countless books by popular authors like Tim Ferris, I realized that although many were, indeed, excellent writers, none of them actually understood the physics they were purporting to explain. At beast, they were simply repeating the metaphors they'd been given. They didn't understand the physics well enough to explain it.
There were a few exceptions- popular books written by actual physicists who also had a particular gift for teaching and explanation. To date, I've only found three who both have a deep understanding of modern physics, and who can convey more than a metaphorical understanding of this to a reasonably intelligent, but non-specialist, reader: Richard Feynman, Alan Guth, and Brian Greene. True, there are other physicists who write popular books, but most aim pretty low. They're satisfied to give a general sort of metaphorical explanation- curved space is like a curved rubber sheet, expansion is like inflating a beach ball, and strings are like... little strings. But Feynman, Guth and Greene each tried to really convey the real science.
The late Richard Feynman is still the master. His lectures- especially "The Character of Physical Law"- did a magnificent job of making clear even such difficult concepts as the quantum explanation of diffraction. Guth's "The Inflationary Universe" does a superb job of explaining topics like tension and negative energy in telling his story of the origins of cosmic inflation theory. And Brian Greene, author of the current volume under discussion, has now produced his third book attempting to explain some very difficult ideas to the lay reader. In "The Hidden Reality", Green tackles string theory, the multiverse, symmetry, group theory, and dozens of other topics, and he does so without resort to any "it's just like..." metaphors. He uses graphic representations when possible, to illustrate mathematical relationships without math when possible (although much of the real math can be found in the appendix.) He explains where and how contemporary cosmological theories originated, and gives the reader a good sense of exactly how we arrived at a position in which physics is largely dominated by untestable theories that make few predictions about the measurable universe- and why this is not necessarily a problem.
Greene is one of the principle authors of modern string theory, and he does a superlative job of conveying, for the lay reader, both the state of string theory, and its genesis. While to fully understand such notions as (say) the role of Calabi-Yu shapes in defining the topology of the multidimensional universe would no doubt require a real familiarity of topology, I think Greene comes as close as possible (or at least as close as I've seen) in conveying to the reader why it is that these shapes play a role in defining space, and how it is that physicists came to propose their existence. His explanation of quantum uncertainly and of Schrodinger's probability wave is probably the best non-mathematical one I've read.
This is not an easy book to read. I went as far as a few calculus courses and a semester of physics back in my undergraduate days, and I found this book fairly hard going. It's not terribly mathematical (except in the appendices) but the concepts are not easy, and there's little if any fluff to be found. This is not the sort of breezy reading found in the typical popular physics book (here's the atom, here's a quark, wasn't that cool?) The reader who attempts to simply skim through without trying to follow Greene's narration and really understand what he's trying to explain will quickly find themselves lost, reading words without a clue of what they mean. I've been reading it for two weeks, attacking a chapter (or part of a chapter) each day, and often backtracking to make sure I understand what Greene is trying to convey. The reader who is prepared to take this approach, and spend a lot of time reading, pausing, think about what they've read, and rereading each section to make sure they really understand what's going on, will find this a very rewarding book.