Take Gowers's delightful little book, "Mathematics: a very short introduction", make it about twenty times as long, bring in a host of excellent contributors to write specialized articles, put the whole thing together very nicely, and you have the present book.<br><br>This book is not an encyclopedia, but it does offer a sweeping panorama of mathematics, written at an accessible level.  It includes introductory articles on what mathematics is and basic concepts, more advanced (but still accessible) articles introducing  various key concepts and areas of mathematics, articles on history of mathematics and biographies of mathematicians, descriptions of key theorems and problems, essays on the applications of mathematics, and more.  There is something in here for everyone with an interest in mathematics.<br><br>As a professional mathematician, I am familiar with most of the introductory material, but I still like seeing it so nicely expressed and might use it as a teaching resource.  Among the more advanced articles, there is lots of material which I feel like I "should" know, but actually don't.<br><br>The editors did an amazing job of finding really top-level people to write the specialized articles, who are both renowned experts in their areas and excellent expositors.  The quality of the writing is infinitely superior to most articles in wikipedia or other online math encylopedias.<br><br>As I said, this not a comprehensive reference.  The articles are introductory and designed for "bedtime reading".  (Although if you read this book in bed you will probably have to sit up and put it on your lap because it is as big as a phone book.)<br><br>Anyway, I was very pleasantly surprised when I received this book.  I expect to spend lots of time in the next few months browsing through it to brush up on my basic mathematical literacy.  I think it will be even more useful for undergraduate mathematics students who want a good overview of what mathematics is about.<br><br>UPDATE: There is a useful page of errata, and discussion thereof, on Gowers's weblog.