I'm shocked no one else has reviewed this book. I believe I heard of it through P. Woit's blog about particle physics, he wrote the infamous anti-string tome 'Not Even Wrong'. The book is written by a philosopher but the understanding and lucidity of his analysis is extremely rewarding to any scientifically educated individual. Each chapter discusses a possible 'theory of everything' starting with the 19th century and moving forward, dealing mostly with 20th century potential grand unified theorems. In this respect most of the book reads as a (recent) history of physics work. For ex. he discusses the 'bootstrap' idea of the 60s for unifying the nuclear forces, which fell through when the quark theory gained ground, from the point of view of the time when it appeared: to those scientists working on it at the time, it could very well have turned to be the 'correct' unifying theory, unfortunately now with hindsight it obviously wasn't meant to be. What are some of the characteristics of these failed theories? The tendency for the people working on it to consider one simple idea the basis for an entire universe-view, the quasi-religious fervour they bring to their theories, the divorce from actual evidence and experiment. So the lesson for our current theories, e.g. strings, loop quantum physics, is obvious. Some of the older theories which were abandoned had just as intense a following as string theory does today. Is it even possible to arrive at a 'universal theory' which explains everything from particle physics to cosmology? Despite the faith of physicists, most people who are non-physicists are very very doubtful. It seems rather to be a peculiarity of the human mind that one wishes to oversimplify and reduce all of reality into a simple (or at least all-encompassing) idea. Note that the author does not get very deeply into the actual philosophical issue of whether it is even possible to reduce the universe into one theory (other books such as 'the limits of science' or Barrow's Impossibility deal with this very well). With regards to this he seems to be unconvinced one way or another though the historical lesson adds up very clearly to an answer in the negative.
This is a book I will definitely have to read again soon, within the year, since it is so dense with ideas and facts of interest to any scientifically educated individual or philosophically minded one.