Best summary : Writing for the Web (Chambers Desktop Guides) by Susannah Ross (only available in the U.K., but it's worth the extra shipping charges to get this book mailed to you from Amazon.co.uk, even if you're in the U.S. or otherwise far away from London.) Ross manages to cover all the most important issues in less space than anybody else, mainly because she focuses on writing , whereas Redish wanders all over many other issues in Web usability (that are important, but covered elsewhere).
I am still looking for the perfect book, so if you do publish a book about Web-writing, send me a review copy and I may list it here if I find it better than these recommendations.
Much is known about how to write help text, online documentation, and other technical writing, and a good deal of the advice from these fields does transfer to writing for the Web. The main difference is that Web readers are much less motivated than readers of online docs since they can't know whether the site is relevant to their goals (in contrast, the docs are always relevant to using a product, even when the writing stinks).
Here are some good references on writing help and online documentation: