A letter to an editor accompanies your manuscript submission. Presumably, you’ve researched and read the submission requirements for the particular editor you are submitting to, and you are following those requirements.
Here is a sample letter that you would put onto letterhead, or into email:
Dear Mr. or Ms. Editor’s Name:
I am submitting my picture book [or whatever kind of book you are submitting] for ages [4-8] titled MY BOOK TITLE IN CAPS. It tells the story of [briefly capture the book synopsis in one or two sentences here.] This is an exclusive submission. [If they do not require exclusive, leave out the sentence, or say it is exclusive until a particular date.]
We met at [the name of the conference or other place where you met, even if briefly. Or, if you didn’t meet but heard the editor speak, mention that. The idea is to then explain why you are sending your manuscript to this editor. There must be a reason – it shows that you’ve done your homework.]
I am a member of SCBWI and have previously published in [say where you were published and give a title or two. If you haven’t been published, state any relevant memberships or education. Include a sentence or two about why you wrote the book or why it fills a need in the market. Keep it brief.]
Should my submission not meet your current needs, I have enclosed a SASE for its return. [Be sure to read the submission requirements. If they don’t want a SASE, don’t enclose one. Or say that the manuscript may be recycled. If you are submitting via email, leave out all references to a SASE.]
Thank you for your time and consideration. I look forward to your reply.
Enclosures: Your manuscript title in italics, SASE [if submitting via email, leave this out.]