This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
Use a cover letter if the work was requested by an
editor (this reminds the editor that he or she wants its).
Use a cover letter if you are submitting part of a
larger work (i.e., sample chapters and outline of a
completed novel.) In a sentence or two, give the title,
genre, and length of the book.
Use a cover letter if your work needs special
explanation. If your article is time-critical (a piece on
the upcoming election primary that will not be useful if
it's shunted aside for a few months), note this in your
Use a cover letter to introduce yourself and list
any *pertinent* information about you and your
accomplishments, if you are submitting "over the transom"
(i.e., without having obtained a request from the editor.)
"Pertinent" means any major writing successes, any
professional or extraordinary expertise in the subject of
the submitted work, or any fame that you may have that will
help sell the work. If you've have best sellers in another
field, mention them. If you are the world's foremost expert
on chair caning and are submitting a piece on chair seat
repair, note that in the letter. If you are submitting a
Young Adult novel about adopted children and you have seven
adopted kids, mention them.
Be brief. Do not mention anything that doesn't
directly highlight your story or article. Don't tell your
life story. Don't retell your story. Don't gush or ramble.
Be concise and professional.