*It's About Time: Understanding Einstein's Relativity*, by N. David Mermin, 2005.192 + xv pp., illustrations, $29.95 cloth. ISBN 0-691-12201-6 . Princeton University Press, 41 William Street, Princeton, NJ 08540, http://www.pup.princeton.edu

Einstein's theory of special relativity solves a nagging puzzle about the speed of light: Why does light emitted from a moving source have the same speed as light from a stationary one? (In other words, why is *c* + *v* = *c*, where *c* is the speed of light and *v* greater than 0 is the speed of the source?) His solution is stunning: (1) just accept the equation as a law of nature; (2) recognize that "time," which is involved in calculating those baffling velocities, cannot be absolute. A moving clock must tick more slowly than an identical one that remains stationary. A book about relativity will therefore be a book about time. The subject is accessible to a high school student who knows algebra and who has the intelligence and perseverance to grapple with novel ideas that challenge the imagination. This is David Mermin's avowed audience, and he speaks to it with consummate skill. His book is breathtaking; the treatment is thorough and deft, and the style is informal and intimate. There is an extra measure of force and clarity that Professor Mermin brings to Einstein's ideas - a measure that comes, I believe, from careful attention, over a long career, to the struggles of his own students. You can survey this power in the author's own brief outline of the book and his guide to teaching relativity at the site http://people.ccmr.cornell.edu/~mermin/homepage/tampa.pdf. If you want to learn special relativity, add this book to your library. If you are familiar with special relativity and want to see how to teach it, add this book to your library.

James Callhan, Professor of Mathematics, Smith College, Amherst, MA