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In most of the US, the easiest way to purchase a copy is to walk into your local huge chain bookstore (Borders, BookStar, Barnes & Noble, Hastings, etc.) and look for it; it's usually mixed in haphazardly with the Java books. The spine of the book has a distinctive purple square on top and should be easy to spot. This book may also appear in a display on the end of the aisle at Barnes & Noble, so watch for it.
You might also try looking at computer stores. If your local bookstore doesn't carry it, don't be afraid to ask for it by name and ISBN number (1-57521-141-6). Most stores should be able to order it.
Amazon Books
(Service offered in association with Amazon.com books)