Writing a cover letter is probably the easiest letter to write. It's not a query or a synopsis, but a short, refined, to the point letter of acknowledgement to an editor or agent.
The three basic components of the letter include:
1. A personal note. If you know the editor or agent, state it, if not, move onto number two.
2. State the title of the work, word count, genre (not be confused with category). Is it an adventure, romance, paranormal thriller, contemporary, etc.? Add the category: picture book, middle grade, young adult, adult, etc. Side note, know your categories, if you've written a 20,000 word picture book or a 200,000 word novel, you might want to do some editing first.
Also, some submissions do not require all four descriptions. If you're submitting a board book or picture book, for example, it would be title, word count, board book or picture book. The genre isn't necessary.
3. List only relevant publishing credits. It may pain you to do so, but unless your trek across the Gobi Desert is pertinent to your submitted manuscript, leave it out. However, if you're an expert in the subject of your story, be sure and mention it.
If you can, try to add your voice to the cover with your syntax.
Of course, make sure your cover letter has the proper format (margins, 12 pt. font, etc.), correct name and address to whom you’re sending your letter, be sure to include your contact info (under your name) or in the letterhead and, most importantly, that your letter is free of grammatical errors.
For examples of a good and bad cover letter, click Literary Journal Submissions 101 by