The Chicago Manual of Style, Fourteenth Edition

The Columbia Guide to Online Style by Todd Taylor and Janice Walker 

The Elements of Style by William Strunk Jr. and E.B. White

The Gregg Reference Manual, Ninth Edition, by William A. Sabin 

Microsoft Manual of Style for Technical Publications

Style Meister: The Quick-Reference Custom Style Guide by Lana Castle

WIRED Style:  Principles of English Usage in the Digital Age by Constance Hale and Jessie Scanlon 

Words Into Type by Margaret Skillin and Robert M. Gay

Dictionaries

Merriam Webster's Collegiate Dictionary, Tenth Edition

Webster’s New World College Dictionary

Books About Editing

Copyediting: A Practical Guide by Karen Judd (note: new revised paperback edition released in August 2001)

The Copyeditor's Handbook: A Guide for Book Publishing and Corporate Communications by Amy Einsohn

Technical Editing: The Practical Guide for Editors and Writers by Judith A. Tarutz

Grammar References

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

Harbrace College Handbook, Thirteenth Edition, by Winifred Bryan Horner, Suzanne Strobeck Webb, Robert Keith Miller

Woe Is I: The Grammarphobe's Guide to Better English in Plain English by Patricia T. O'Conner

Web Content Guides

Designing Web Usability: The Practice of Simplicity by Jakob Nielsen

The Internet Writer's Handbook by Martha C. Sammons

Writing for the Web, Geek's Edition by Crawford Kilian