For the past four years when I have taught the non-fiction writing class at the Professional and Continuing Education program at the University of Washington, I have used William Zinsser's book "On Writing Well." It's my preference probably because I agree with nearly everything he says there: Make sure every word is doing work, etc.
He's right in line with my goal of getting the most meaning out of the fewest number of words -- OK, I'm in line with his purpose in non-fiction writing, not the other way around.
All these many words to introduce a New York Times story on what Zinsser, at age 90, is doing now. Have a look.