The Elements of Style by William Strunk and E.B. White

This usage guide is full of good, concise information to help readers improve their English prose. The authors forcefully advocate a stripped-down style, where each word must count. Some of the usage notes for words strike me as overly fussy (which may be a matter of taste or a sign of the book's age), but I think Strunk and White's suggestions are a great starting point for developing a tasteful and sound prose style.