"Google Hacks, Second Edition" is a particularly useful book for many BlogCritics. It provides a how-to approach to doing useful, interesting, and somewhat silly things with Google, the syntax, scripting, the API, GMail and Adwords. With its spare interface, many people overlook a lot of what Google can do.
For many BlogCritics, the most useful sections will probably be using Google search syntax, incorporating Google CGI scripts into your site, and learning how to make most effective use of AdWords and Pagerank. Much of the advice in this section comes down to common sense, create good content, and the web will find it. The section on localizing content, and providing searches relevant to your blog are really helpful at solving problems, which is often overlooked in many computer books.
As with other O'Reilly books, this is a useful resource, however, since it deals with Google and the web, many of the hacks have a limited life-span.
You can get more information about this book at O'Reilly, which also includes the source code for many of the scripts used in the book.