The Web Content Style Guide should be available in most good bookstores. However, if you would like to buy it online, here are some links you might find helpful: 

Amazon.com
Buy The Web Content Style Guide

Amazon (UK)
Buy The Web Content Style Guide

Amazon.fr
Buy The Web Content Style Guide

Amazon.de
Buy The Web Content Style Guide

Amazon.jp
Buy The Web Content Style Guide
 

Other books by Gerry McGovern
Buy Content Critical