The Elements of Style
By William Strunk, Jr. and E.B. White
I mentioned this book in my post on Storytelling Resources, but I thought it deserved its own call out. An incredibly slim book, you can read and understand it in an evening, but will likely return to it again and again. This book clearly lays out the rules of modern American usage and style, favoring concise, vigorous prose. Even if you enjoy waxing more eloquent, this provides a great starting place from which to depart. It's also admirably organized, with a detailed table of contents and clear headings for the rules it propounds, making it ideal as a reference work (which it will be for the vast majority of its usefulness, since it will only take an hour or two to read it the first time). If you write at all for your living or for pleasure, you should have this book in easy reach at all times.
Get it here.