Simply Einstein by Richard Wolfson
Einstein's Theory of Relativity is based on (relatively?) simple ideas. It is just that those ideas have some profound consequences which are unusual and unexpected, to say the least. The simple ideas are: the laws of physics are the same everywhere and everywhen, and light is always measured to be traveling at the same speed, regardless of how it is created or measured. However, as a result we find that lengths and times do weird things when stuff starts moving around the universe.
Wolfson hangs his explication of relativity on the idea that it is based on one simple and sensible concept. In doing so, he tries to convince us that maybe Einstein's ideas aren't so difficult to comprehend after all. Unfortunately, no matter how many times you tell a reader that something is understandable, you can't make somebody understand. And relativity is a topic that is genuinely difficult to understand at a deep level unless you've spent a lot of time thinking about it. Many physics graduates never really come to terms with the ideas fully and it is perhaps too much to expect the lay reader to do any better.
With that in mind, all we can hope for from a popular book on relativity is that it explains the main questions the theory addresses, what sort of resolutions it gives and why we should believe it is correct. We can hope that the book will lead the reader to a deep and fundamental understanding of the topic but we shouldn't be placing bets on it. If you also want the reader to feel all warm and fuzzy, they should come away with an impression of understanding (and a little jargon to throw around at parties so they can show off). A lot of popular science books are good at this latter task, especially when it comes to the big two topics of physics popularization: relativity and quantum mechanics. The truly difficult part is guiding the reader to a deep understanding.
So how does Wolfson do with his explanations? One of the strengths of the book is that it deals with many of the features of relativity more slowly than other popular books on the topic. Wolfson spends a great number of pages on the unusual aspects of relativity such as the idea that an observer will measure an object moving relative to it as shorter than the length measured by somebody moving with it. The idea that clocks carried by people moving relative to us appear to run slow to us is also dealt with at length. These ideas lead to all sorts of intriguing and odd puzzles and paradoxes that are a delight to think about and wonderful to hear resolved but, like much of relativity, there is always more lurking beneath the surface for a careful thinker.
Wolfson only deals with the basic aspects of relativity - the sort of topics often dispensed with in a chapter or two of other popularizations. If you've tried to get through those chapters and had trouble, this could be the book you need. However, if you are looking for more, none of the really fascinating ideas at the forefront of relativity theory are included.
Unfortunately, the book falls down a little on providing the evidence that physicists use to base their whole field on Einstein's ideas. Wolfson does mention some of the experiments that are used to test relativity and he does a decent job explaining them but you'll never feel the strong connection between the theoretical ideas and the experimental evidence supporting those ideas. This would be useful seeing as the theoretical ideas are at odds with everyday experience.
Knowing that experimentalists really have tested this theory in exquisite detail and shown that the universe is truly odd would at least replace the wonder you might have about whether you are being fooled into believing all this abstract weird stuff. (Just to clarify, the universe truly is weird. You just have to deal with it no matter how much you don't want to believe it.)
Some more complicated ideas are treated in the last few chapters when Einstein's General Theory of Relativity (his theory of gravity) is discussed. They are fun stuff and deal with black holes, curved space and the fate of the universe. The less detailed treatment allows Wolfson to keep the tone light as he moves through ideas quickly and effectively. They are the strongest part of the book.
There is, however, one very serious problem with Wolfson's approach. In trying to base everything on one simple concept, he inadvertently hangs his entire book on a circular argument. (Be warned, technical and philosophical comments follow!)
His simple idea is that "The laws of physics are the same for all." While undoubtedly true, this doesn't really capture everything required for Einstein's special relativity. Wolfson claims that this statement is fundamentally different from Galileo's version "the laws of motion are the same for all" but it is hard to believe that Galileo ever thought that other areas of physics besides motion had laws changing from person to person. After all, you can't really "do" physics if you believe the rules are different for everybody. Other authors consider the idea that the laws of physics are the same for all as common to all physics since Galileo.
Einstein's relativity is usually written in terms of two premises: 1) the laws are the same for everybody, and 2) that there is a maximum speed limit in the universe (alternatively phrased as "the speed of light is always measured to be the same regardless of the motion of the source or observer"). This second premise is absolutely essential to differentiate Einstein's universe from Galileo's and Newton's conception of the world.
Wolfson claims that the second premise can be dispensed with. Unfortunately, you can only do that if you make the implicit assumption that the world must be how we measure it. That might seem like a fair assumption to make, but the whole point of physics is to find the set of laws that matches our universe in among the many different sets of laws that would lead to similar but different universes. This is what makes Wolfson's claim circular. He essentially says that given the universe is how it is, then Einstein's relativity must be true. But seeing as the universe does have Einstein's relativity built in, the claim is circular.
Here is another way to look at it. Consider there are two different sets of possible laws for the universe, Galileo's and Einstein's. Einstein says there is a maximum speed limit in the universe (the speed of light) while Galileo says there is no fundamental maximum speed. Both sets of laws are fully consistent with the idea that the laws of physics are the same for everybody. So to distinguish between these two possible universes, we need some extra criterion - is there a maximum speed or not? As the relativist Wolfgang Rindler points out in his textbooks, the entire role of the second premise, that of the constancy of the speed of light, is to distinguish between a Galilean universe and an Einsteinian universe.
To resolve the issue and work out which type of universe we actually live in, we have to do an experiment. When we do that experiment, we find that the universe is indeed Einstein's. Wolfson builds this finding in as a necessary part of the universe, a claim that isn't strictly warranted.
Perhaps this philosophical issue wouldn't matter so much but Wolfson keeps on harping on the idea of everything coming from the single principle and it just isn't true. It seems that in an effort to make relativity accessible to everybody he has simplified the story beyond the point where it remains accurate.
So if you do read this book, you're going to have to try to ignore the incessant claim of a single simple concept as the foundation for relativity. Fortunately, the details of everything else are fine and often far better explained than in other books.
Relativity is a fascinating idea that is immensely rewarding to understand. You just have to be prepared to do a little work and grapple with ideas that defy intuition and common sense. Beware of oversimplification because it might just lead you to an inaccurate understanding of the physics.
Simply Einstein: Relativity Demystified by Richard Wolfson
W.W. Norton & Company