I actually have two copies of this now; a very concise and clear guide for any kind of nonfiction, including research papers, news articles, web articles, etc. This book has good basic rules that can apply to almost anything; your wrting will improve over all.
Kate M. (Kinare) reviewed On Writing Well: The Classic Guide to Writing Nonfiction on + 6 more book reviews
This book is essential for the working journalist who is interested in entering the narrative nonfiction style of writing. It has helped me tremendously with my writing. I also suggest Writing for Story by Jon Franklin.
The authoritative guide.