General Writing Guides

Clear and Simple as the Truth
by Francis-Noel Thomas and Mark Turner
Thomas and Turner offer guidance to writing with authority.
The Elements of Style
by William Strunk and E.B. White
This is, simply put, the finest guide to clear and concise writing that you can find. Read it.
The Everyday Writer
by Andrea Lunsford
Lunsford offers a variety of useful lists and guidelines in this packed little tome.
Style: The Basics of Clarity and Grace
by Joseph Williams
While I have not read this, some professors swear by it.