The [b][i]Source[/i][/b] :shock: for clarity and brevity is William Strunk's The Elements of Style. It seems every professional author and writing class considers it indispensible. It's as useful now as it ever was.
It was updated some years after initial publication by E.B. White, one of Strunk's students, but the original is now public domain. You can find it online here in HTML format:
Or if you prefer your own copy, you can download it in Acrobat format from the web. I have a copy I can email anyone who is interested.
Or you can buy White's revised version in any bookstore. It's on it's fourth edition. That version is not public domain. It goes for around $9 softcopy at Barnes and Noble. Or you can get it at Amazon:
http://www.amazon.com/exec/obidos/tg/detail/-/020530902X/qid=1060906374…