I highly recommend Strunk & White to anyone interested in writing, because it gets to the heart of what I believe good writing is: Sharp, well-placed words and as few of them as possible. Clunky phrasing and show-off SAT words are signs of weak writers, and attempts to impress the reader only water-down the message.
But for people who can't be bothered to read an entire book, there's George Orwell's famous essay, Politics and the English Language. Orwell carves out the same message but in a smaller space.
Well now I've seen this Calvin and Hobbes comic and I have a new alternative to people who can't sit still enough for Orwell.
It's from 1993, but the message is timeless.