This book is the story of the universe and the bit. The universe is the biggest thing there is and the bit is the smallest possible chunk of information. The universe is made of bits. Every molecule, atom, and elementary particle registers bits of information. Every interaction between those pieces of the universe processes that information by altering those bits. That is, the universe computes, and because the universe is governed by the laws of quantum mechanics, it computes in an intrinsically quantum-mechanical fashion; its bits are quantum bits. The history of the universe is, in effect, a huge and ongoing quantum computation. The universe is a quantum computer.
Basically, Seth Lloyd's message is that information is just as important as classic physical quantities like energy in understanding the universe, and that viewing the universe as a quantum computer can actually help us understand it better. He even proposes a theory of quantum gravity based on this view---a theory that, unlike others, may even be testable experimentally.
You might like to have a look at a scientist's cut from the material Lloyd penned down for the book here. There you will also find some interesting quotes from referee reports Lloyd received for some of his papers, and the following estimate on the number of operations performed by the universe since the big bang.
The universe can have performed 10120 ops on 1090 bits (10120 bits including gravitational degrees of freedom).
I wonder if Scott Aaronson has reviewed this book on his blog. It would be interesting to hear his opinion.