This book was recommended by a fellow HN'er a few years back in a different thread. I bought a copy and read it and was suitably impressed. I'm still working on integrating the ideas from the book, but I think it's worth reading.
Basically, the book teaches you to organize your thoughts (and writing) in a hierarchical, logical structure, and to present the most important idea first, and then branch out below that with sub-points and supporting material.
If you're interested in clear writing, I think this book is worth the money and time.