I second On Writing Well. It's an absolute must read for anyone writing non-fiction. There are two other books I'd throw up with it—Revising Prose by Richard L

I second On Writing Well. It's an absolute must read for anyone writing non-fiction. There are two other books I'd throw up with it—Revising Prose by Richard Lanham and The Elements of Style by Strunk and White. Those three books together are a fantastic course in writing.