Style, Usage, and Documentation

Websites

While the MLA website does not publish its style sheet online (they prefer that you buy their books), the site does have an extremely helpful page of FAQs.
The APA website contains a helpful FAQ section on the mechanics of APA style as well as particular grammar issues.
Barteby.com has exerpted from Strunk and White’s Elements of Style. The site contains helpful information on usage and composition issues.

A Pocket Style Manual
Diana Hacker

English Grammar in Context
Kitty Chen Dean

The Imaginative Argument: A Practical Manifesto for Writers
Frank L. Cioffi

Prentice Hall Reference Guide
Muriel Harris

EasyWriter: A Pocket Guide
Andrea Lunsford and Robert Connors