A Snarkling takes the next step:
Say an agent requests a partial from a query letter. What sort of things are good to include in the cover letter that goes with the partial? Do you blurb the book and put in your credentials again, like in a query letter, just adding a line mentioning that they requested the partial? Or should it be something different altogether? Everyone talks about query letters, but you never hear a whisper about cover letters once your query letter worked.
You can include a copy of your actual query letter to remind the agent why s/he found you enticing. The cover letter can then include info you might have left out of the query: word count, whether it's being considered elsewhere. If you re-read your query you might find some other points to cover but succinct is a virtue here.
Make sure you say what you're sending: first three chapters or first 100 pages. Despite my rant below about following directions, if a chapter breaks at page 98, I'd send only 98 pages and tell them so in the cover letter. (ya ya ya, consistency is the hobgoblin of small minds--well Miss Snark has a big head, we all know that).
Remember- Don't tell an agent how long s/he can have it exclusively. Don't send any partials on an exclusive basis unless the agent asks and then it should be two weeks or less for under a hundred pages. Agents who want to tie up your work for longer than that aren't acting responsibly. You can quote me on that. If an agent does want an exclusive, you can say “I’m confirming you asked for this exclusively for two weeks; until December 25, 1902” if you want to be clear. Clear is good.
Also, don't give deadlines. If you're leaving the country for an extended trip, say so, but do NOT say "I'd like to hear back before I canoe across the Pacific with my new spouse Mr. Ahab".
Don't forget to include a #10 SASE on a partial.
A good closing is: “Thank you again for your time and consideration. I look forward to hearing from you.”
Then, pop it in the mailbox and dance for joy! Send pics of the dance part!