AllyWoof, by "cover letter" I assume you mean a letter which accompanies a story, novel, article, or other submission, right? And I also assume you know that for novels, you send a query first, not the manuscript?
Good. You seem smart already!
A cover letter just tells the agent/publisher what you're sending and reminds them that they asked for it, if that's the case.
Enclosed you'll find my 110,000-word fantasy,
Dinkdorfer, as requested in your letter of April 31, 2009. I hope you enjoy it and I look forward to hearing from you.
or
Enclosed is my mystery story of 2500 words,
Cat on a Hot Tin Can, which spoofs the classic play and movie. You can reach me at
[email protected] or use the enclosed SASE.
or
Please find my article
Making Do With More on submission exclusively to PublicationName until September 1, 2009. Thank you for your consideration.
Short, direct, with little 'sell' text, since the enclosed material will speak for itself.
Maryn, trying to be helpful but probably blowing it