Here are some useful sources of information:
General query letter resources
Write Query Letters That Sell by Michelle Lee, who has worked as an editor at Mademoiselle, Us Weekly, CosmoGIRL, Parenting, and Glamour magazines. She’s written for Self, Marie Claire, Cosmopolitan, Ladies Home Journal, Fitness, Fit, YM, Teen, Maxim, Men’s Health, and others.
How To Write A Successful Query by Moira Allen. An excellent article, part of Writing-World.com.
How To Write A Query Letter by John Hewitt. I especially like the section on what NOT to put in a query letter.
Publish Or Piss Off: Query Letters by Linda Formichelli. Column written for 1099, a magazine for independent professionals.
How to Write a Query Letter: part of eHow.com.
Writing A Query Letter by Charlotte Dillon. Excellent resource, with useful articles, links and sample query letters.
Query Letters by Tara K. Harper.
Writing A Query Letter That Sells by Alex Keegan.
The Complete Nobody's Guide To Query Letters by Lynn Flewelling. Part of SWFA.org.
Dos and Don'ts: How to write the perfect query letter by Gail Eastwood.
Write A Query Letter That Sells by Marilyn Henderson.
Query letters to a literary agent
SoYouWanna write a query letter to a literary agent? (all except children's books) Includes a sample query letter.
Books about writing query letters
Writer's Guide to Queries, Pitches & Proposals by Moira Anderson Allen. Excellent resource, written by the editor of Writing-World.com. Includes useful info about online queries, tips on how to approach newspapers with column or syndication ideas, how to get corporate freelance work, other opportunities.
How To Write Irresistable Query Letters by Lisa Collier Cool. This was the guide I used when I was just starting out; it remains a good resource. Focus is on pitching magazine articles, though there is a chapter on how to query agents and book publishers.
The following are books I found in Amazon but haven't read myself (if you've used any of these, please do post comments):
How to Write Attention Grabbing Query & Cover Letters by John Wood.
The Renegade Writer's Query Letters That Rock : The Freelance Writer's Guide to Selling More Work Faster by Diana Burrell and Linda Formichelli.
ive 'Em What They Want: The Right Way to Pitch Your Novel to Editors and Agents, A Novelist's Complete Guide to : Query Letters, Synopses, Outlines by Blythe Camenson and Marshall J. Cook.
Query Letters That Worked!: Real Queries That Landed $2K+ Writing Assignments by Angela Hoy.
Writer's Guide To Book Proposals: Templates, Query Letters, And Free Media Publicity by Anne Hart.
Do you have other suggestions for useful resources? Please post them below!