The Elements of Style by Strunk & White, illustrated by Maira Kalman

Even though some of these rules are largely ignored in modern American English, this is a valuable guide to avoiding common mistakes. It covers the basics - "your tool" / "you're a tool!" - before moving on to more general refinements of style. My favorite bit of advice: "Do not dress words up by adding -ly to them, as though putting a hat on a horse."