Editors on Editing: What Writers Need to Know About What Editors Do, Third Edition. Edited by Gerald Gross. A collection of illuminating essays by distinguished editors.
Educational
Copy Editor Workshop page. A very useful set of annotated lists of workshops, including online, on-site, and correspondence courses.
Miscellaneous
Authorlink! An extensive site featuring news, resources for writers, editors, and agents, a manuscript marketplace, publishing imprints, classifieds and more.
The Chicago Manual of Style FAQ. Questions and answers from the manuscript editing department at the University of Chicago Press, publishers of the venerable CMS.
Copyediting-L Archives. Popular mailing list devoted to copy editing issues.
Copy Editor. A print-only publication aimed at newspaper, magazine, and book copy editors. Free sample issue available. Also has job board, links, and workshop information.
WWWebster Dictionary. Merriam-Webster's Collegiate Dictionary, Tenth Edition. Considered a definitive dictionary. Searchable, with a thesaurus.
William Strunk's The Elements of Style. The 1918 text of Strunk's masterpiece. Useful, but buy Strunk and White's latest print edition. It's cheap and invaluable.
A Web of On-line Dictionaries. Links to hundreds of dictionaries in more than a hundred languages.
Wired Style. A slightly snooty but helpful site from the arbiters of Internet Hip.
Professional Organizations
American Society of Indexers. "A nonprofit educational and charitable organization, ASI serves indexers, librarians, abstractors, editors, publishers, database producers, data searchers, product developers, technical writers, academic professionals, researchers and readers, and others concerned with indexing."
The Freelance Editorial Association, based in Cambridge, MA, is a nonprofit membership organization staffed by volunteers. It works to promote the interests of freelance editors, illustrators, indexers, production specialists, proofreaders, translators, and writers. Very useful site.
Freelance Online. Aimed at freelancers in publishing and advertising. Features job board, directory, forum, bbs, and more. Some features are free, others for members ($15 fee to join).
Editors Association of Canada. A national, nonprofit organization of both English- and French-language editors, with more than 1000 members from coast to coast.