Greene is a good writer, and does his best to simplify the science behind these complicated theories However, it's a daunting task: string theory in particular is so abstract and antithetical to our everyday life that it's very hard to follow - particularly if you're reading the book in segments. For example, here's an interview where he attempts to explain the brane multiverse:
...the brane multiverse, in which our universe is envisioned to reside on a giant membrane, an ingredient that comes out of string theory. It’s actually a three-dimensional membrane, but thinking in two-dimensional terms is easier. Think of our universe as if it were a huge slice of bread, with all the stars and all the galaxies sprinkled across its surface. The math of string theory suggests this picture, along with the possibility that there are other universes, other slices of bread, all constituting a big cosmic loaf.In the book, he expands upon the three-dimensional idea by stating that "...few of us can picture two coexisting but separate three-dimensional entities, each of which could fully fill three-dimensional space." (page 130) His loaf analogy is a nice attempt, but he has to keep revisiting it whenever new details arise, until the whole thing gets incredibly complicated - just like the theory. For that reason, I can't recommend this book to the layman; it's just not an easy-to-grasp, high-level explanation of the concepts behind parallel worlds. It is an interesting book on a fascinating subject, but in my opinion it couldn't get over the hump that simply grasping the meaning of theoretical physics can be difficult, much less following them to their logical conclusion.
Note for the Kindle edition: I found the footnotes and graphics to be problematic, mainly because you can't jump back to where you were without making note of the page number and manually entering it in. Would it really be that hard to make the footnote a two-way street? Also: there was obviously no color in the graphics. These two factors combined made me wish I had purchased the dead-tree edition of this book.