Welcome to our universe. We only get one (regardless of however many there are). The search for a more complete understanding of our universe, out into the macroscopic and deep into the quantum foam, is a search for an understanding of who we are, why we're here … and where we might end up. This is a book of sublime thought that takes the ivory tower and turns it into an ivory ladder that anyone, given inclination and opportunity, can choose to ascend, one rung at a time. I cannot emphasize enough how important this book is to physics: it's a Philosophiæ Naturalis Principia Mathematica for the masses.
Unlike Newton's groundbreaking scientific treatise, A Brief History of Time doesn't contain Hawking's own body of scientific work, but rather an overview of the development of theoretical physics, including relativity and quantum mechanics. Yet it's as important as the Principia, for in a single book we have a comprehensive look at a field of study often considered by the general populace to be obscure and esoteric. In a few hundred pages, Hawking demonstrates why we should be interested in the universe. He explains how relativity overturned the classical theory of Newtonian gravity, how quantum mechanics has exposed the flaws in relativity, and how physicists continue to search for a theory of quantum gravity to unify relativity and quantum mechanics in a Theory of Everything.
Relativity and quantum mechanics are the foundations of physics, chemistry, and biology as we know them today. While A Brief History of Time cannot, obviously, serve as a detailed explanatory text of every phenomenon, it acquaints the reader with the two fields that underly all phenomena, from optics to cell division. Reading this book gives you understanding that will help you with future intellectual inquiries.
Even if you're not interested in science, however, and have no intention of going further than this book, there's still something in here if you've ever wondered how the universe works. Hawking does not deliver a dry lecture consisting of complicated formulae and logically-implacable mathematical theorems. There are new terms, and some of the concepts might seem counterintuitive, but Hawking always has an analogy or concrete example at the ready. I won't claim that you'll understand everything he discusses—I know I didn't. And, as Hawking points out, even the most brilliant scientists still don't have a complete understanding. At the very least, you'll have a much better appreciation of what we don't understand, and why current scientific theories about the universe work but still have certain problems.
I am immensely grateful to my grandparents, who gave me this book as a Christmas gift, for its illustrations make it superior to previous editions. Utility aside, let's be shallow for a moment: the illustrations make the book so beautiful. This is a true coffee-table book (and probably, for many people, that is all it will ever be, sadly). It's well worth reading, but it's also perfect for keeping in the living room—you can always open it up to an interesting illustration and show off your physics knowledge!
In fact, I would go so far as to say that understanding these two concepts (that there may be more than four dimensions, and that curvature in three dimensions is a straight line in four or more) contributes to an understanding of the majority of this book. The barrier here is one of geometrical conception and not physics knowledge; i.e., you don't need to be able to solve its equations to understand relativity.
Some of the illustrations are somewhat redundant or even confusing. Others are invaluable supplements. For example, both of the books hinge on the idea that the universe has more than three dimensions: there's at least four (spacetime), and probably more like 11 or 26. Now, when Hawking uses the existence of these extra dimensions to explain how relativity results in the curvature of spacetime or why gravity is weaker than it should be, it makes sense—but we can't visualize it, because it's impossible to visualize any more than three dimensions. The illustrations depict four-dimensional space as a three-dimensional diagrams (with a spatial dimension removed and replaced by the time axis), which at least gives a better idea of what Hawking means by, "The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space."
This book isn't perfect. Hawking's original treatment of time travel, for instance, leaves much to be desired. He rectifies this in The Universe in a Nutshell, providing a much more comprehensive look at how general relativity might allow time travel. Yet other parts of the second book heavily retread what Hawking discusses in A Brief History of Time, to the point of using similar or identical examples. This is not surprising, considering the two books were published separately. My advice is that if, like me, you read these books back-to-back, then skip over any parts of The Universe in a Nutshell that you like. Even Hawking admits in the foreword that the book is designed to be less linear than A Brief History of Time; delve into those chapters that interest you and don't worry too much about reading every single word on every page.
Regardless of how one reads it, A Brief History of Time should be required reading. As its track record indicates, it has well served its purpose as an accessible physics text. This is a book that presents theoretical physics as a comprehensible, cohesive conversation between Hawking and the reader. This edition, with its illustrations and the inclusion of a second book, The Universe in a Nutshell, is perhaps the best edition of the two books published to date.