Cover letter for a partial?

It's just good form to include a submittal letter. A single line will do, and it's best if you mention that the material was requested, both in the letter and on the outside package. Makes things tidy and business-like.

Enclosed please find x pages of my novel TITLE as you requested in your letter of 10/31/07.

Yeah, don't feel like you have to write all that much. As for me, I just thanked them for their response and interest in y-book (i.e. my young adult zombie book TITLE), indicated that I was enclosing/attaching what they wanted per their request on x-date and asked that they let me know if they had any problems opening it (if it was an attachment). I mentioned that I was also enclosing the original query and that I looked forward to hearing from them and thanks again for their time and consideration.

To me, the points to get across without wasting their time were:
-- you asked for my work, here it is
-- a reminder of what it was they asked for and who I was and what my work was (cause they see so much, I just wanted to make it easy for them to remember my book).

Short, sweet, and remind them that they are free to contact you at any time...

Regards,

and

<15 lines of every possible way they could reach you.>

"Here's the manuscript/partial you requested. Thank you very much."

I've only had to do one of these letters once (sniff sniff), and they had also requested a synopsis. Here is what I wrote:

Thank you for the opportunity to send you a synopsis and two chapters of my novel, entitled What’s Hot. As I am sure all writers tell you, it is very difficult to distill a novel into a two-page synopsis without feeling as though a great deal has been lost. However, I hope the first two chapters will give you a good sense of the tone and sensibility that prevail throughout the book.

I would, of course, be happy to send you the full manuscript or anything else you would like. Please don’t hesitate to contact me with any question or request. I look forward to hearing from you.

Click to expand...

They asked for the full, so I guess my letter wasn't totally offensive.

Seriously, you already sent her the query; it did its job and now you have a request. Why would you repeat it? The agent is not stupid, and don't treat her like stupid. Mark the envelop with "Requested Material." The purpose of the cover letter is simple: Here's what you requested, thank you very much for your consideration.

Be neat, concise and don't waste the agent's time.