farther or further?

Eats, shoots and leaves
This bestseller from 2004 become an instant success. Almost everyone has a copy of the book by Lynne Truss at home or at work, but did they ever read it? Find out who dared give it a savage review, and why.

The Economist style guide
Does the team from the Economist deliver the same level of in-depth analysis in this book on English style as it does in the leading weekly?

The Cambridge encyclopedia of the English language
It may be thick, but just how well does this all-encompassing book on everything do do with the English language really stack up?

English usage: farther or further?