The Elements of Style (Strunk and White)
This is the classic that belongs on every writer’s bookshelf. Seriously, if everyone who writes in English would read and follow the advice given in The Elements of Style, the world would be a better place (and I would be unemployed). This short, inexpensive little gem provides a usage guide, a section of commonly misused words, and a very practical “Approach to Style.”
Some of my favorite bits:
“Use definite, specific, concrete language.”
“Write in a way that comes naturally.”
“Be clear.”
If you don’t own The Elements of Style, buy it. If you own it but haven’t read it in years, dust it off and re-read it. You will be a better writer for having done so.
The full text of the 1918 edition is available online at http://en.wikisource.org/wiki/The_Elements_of_Style