In "How to Build a Time Machine", Paul Davies, Professor of Theoretical Physics and a veteran of the popular science writing genre, has produced a delightful little book about time and time-travel. The format of the book is reader-friendly, written in his usual clear, lucid and engaging style with text linked to relevant sketches, photographs and diagrams of machinery. At the heart of the book is an explanation of Einstein's theory of relativity and what that theory tells us about the possibility of time-travel. Einstein's theory tells us that travel into the future is certainly possible while the possibility of travel into the past has not yet been ruled out. What makes this book such a fascinating and fun read is finding out about the practicalities of building a time machine. If we want to travel into the future, Davies tells us, all we need do is build a machine that moves fast enough. Any machine will do so long as it can move at a velocity close to the speed of light. Things become really interesting however, when we start to think about how to travel backwards in time. The method Davies outlines in the book involves using a wormhole adapted to form a time machine. You jump into the hole and come out in another place and in another, past time. This is a "machine" that is part of the structure of the universe, a machine you step through into the past. Davies then presents a step-by-step guide to building such a machine with illustrations of the various components and descriptions of the processes involved before discussing some of the paradoxes of time-travel. A more interesting way of learning about Einstein's theory than this is difficult to imagine; this is a highly entertaining read and an excellent introduction to the subject of theoretical physics. --"Larry Brown"