This paragraph is where you set the context for your submission. Did you meet the agent at a conference? Are you responding to a manuscript call? Were you referred by someone else? What this is not the place for is explaining how this book was written for your child/grandchild/niece or to explain that these are the true exploits of your most amazing and adorable cat. No matter how true these things may be, I don't need to know them, and they will mark you as an amateur.
- One-Two paragraph pitch
The next one or two paragraphs should be your pitch of your book. Like a published book's jacket copy or a written, more detailed elevator pitch, this is a teaser that gives the overall major plot arcs of the story, a feel for the major characters, the genre and age range of the book, the themes you tackle, possibly the setting (if important), and anything else you feel is important and will help set your book apart from the other comparable books out there. You are not quoting or paraphrasing the text, merely summarizing, but if possible you should still try to convey your voice -- your own distinct writing style that makes your writing sound like you.
(*NOTE* These are very difficult to write but extremely important to get right. If this paragraph(s) does not interest the editor/agent reading it, there's a very good chance the rest of the letter and your manuscript will not get read either. This is not to paralyze you into incapacitating writer's block inducing fear, but merely to make you aware of the importance of a good pitch.)
- Series pitch
If you see your book as the beginning of a series, this paragraph is the place to tell me about it. However, if you don't see this as a series (and despite the tale bookstore shelves may tell, not every book is the first in a series), do not suddenly try to develop one for your cover letter. Just skip on to the next paragraph. Besides, if your editor/agent ends up seeing it as a series, they will be happy to tell you. You'll then be left with the pesky little detail of trying to think one up. Worry about it then.
This is the place to tell me a little bit about yourself. But be professional here. I would like to know if you have a PhD in literacy or an MA in Children's Literature. I don't want to know that you've been reading children's books since you were a child. Also, this is the place for any professional associations that you belong to like SCBWI, Writer's Leagues, etc.
Since we are discussing cover letters for unsolicited manuscripts (and yes, answering a manuscript call is still technically an unsolicited manuscript), you would then thank the agent/editor and sign the letter. However, if we were discussing queries, this would be the place to politely ask to submit the manuscript.
For a great discussion and annotated query letter, see Brooklyn Arden's post from a year ago. Although a query letter, all of the pertinent information is the same. I greatly admire this editor, and if you don't already follow this blog, you should consider doing so. She is a great resource of information.