I read The Fabric of Reality by David Deutsch. With Brian Greene's similarly-titled book still fresh in my mind, when I read the words "theory of everything" I was expecting a book about the implications of a quantum theory of gravity. But Deutsch's aims are far grander than that. He imagines a "theory of everything" in which we can identify common principles that apply to everything from fundamental physics to biology, computation, or economics.
Deutsch rightly makes a point of trying to change scientific dialogue to center around explanations rather than predictions. After all, any idiot can come up with a theory that is consistent with observed evidence (cough, creationism, cough). Instead, we have to judge theories by their explanatory power. In the "theory of everything" as he imagines it, Deutsch does not picture all knowledge as being explained in terms of fundamental physics, because an explanation which crosses too many levels of abstraction is not illuminating at all. Rather, Deutsch makes the case that we can uncover general principles that connect apparently disparate fields, and that such links are the only way we can really comprehend the universe. He gives examples of principles from quantum physics, evolution, epistemology, and the theory of computation— what he calls "the four strands"— that can help us understand all of the others.
I think the vision is a good one but its execution here is just too muddled. According to Deutsch the multiverse (many-worlds) theory is the only sensible interpretation of quantum mechanics. But he really just hand-waves his way to this conclusion without making any compelling arguments in its favor. Now, I'll grant that there is ambiguity surrounding the "correct" interpretation of QM. But it's still a huge leap from there to claim, as Deutsch does, that the multiverse theory has implications for morality, ethics, and free will. I just don't buy that.
What really bothers me about this book is Deutsch's highly specific and fantastic extrapolation. He asserts vast generalizations of principles such as the Church-Turing thesis, but his justifications seems to be nothing more than wishful thinking. Deutsch does advertise his claims as speculation, but it seems that he is falling into one of the traps he himself warns against: that is, believing that pure logic can make any meaningful statements about the physical universe.
There are many interesting nuggets in this book, and it is certainly intellectually challenging, but on the whole I wouldn't recommend it.