Essential resources for writers and editors:

The Chicago Manual of Style: The Essential Guide for Writers, Editors, and Publishers. The bible of book, magazine, and business editors. Get the 16th edition.

The Associated Press Stylebook and Briefing on Media Law
(used to be the Stylebook and Libel Manual, which looked much more, um, impressive on the bookshelf). Required companion for newspaper and magazine writers and editors.

Barron's Business Guides: Dictionary of Computer and Internet Terms. Finally, a good style guide in this area.

For everyone:

Strunk & White's The Elements of Style. Your English teacher was right: Buy this and use it.

On Writing Well: An Informal Guide to Writing, by William Zinsser. A good guide to nonfiction, with a focus on clarity. I recommend this book to my students and clients.

Woe Is I, by Patricia O'Conner. Professionals, amateurs, people writing grocery lists: everyone should buy this clear, simple manual for using the English language.

The Deluxe Transitive Vampire: The Ultimate Handbook of Grammar for the Innocent, the Eager, and the Doomed, by Karen Elizabeth Gordon. Very funny and very, very useful.