The Classic Guide to Writing Nonfiction.
In On Writing Well, William Zinsser practices what he preaches, writing engagingly about writing engagingly.
Some of the stuff, you'll know about already: cut back on the adverbs; keep practising; if in doubt, two short sentences are better than one long sentence; that sort of thing. But there's plenty of other food for thought in this enjoyable volume.
Highly recommended.