A query letter is very important, particularly if the editor does not know you. If the letter doesn’t grab the editor, he won’t read on or ask for more.
When I teach a basic article writing class, this is what I say the query letter should include:
1. Why are you qualified to write this article?
2. What is it about (told in one paragraph)?
3. Who is your audience and why will they be interested?
4. Why will this book be appropriate for this particular publication?
For books, the cover letter is every bit as important as a query letter, so it should be able to stand alone. If a publishing house is considering your book, most of the committee will not get beyond reading the cover letter.
The Cover/Query Letter should basically answer four questions:
1. Why are you qualified to write this book?
2. What is it about (told in one paragraph)?
3. Who is your audience?
4. Why will this book be marketable?
If your article or book topic calls for statistics, they should be included in your cover letter also.