Do your research, whether you are writing a query letter to an agent or to a publisher. If an agent is looking for women's fiction only, don't send that agent your thriller. If a publisher or editor is looking for nonfiction picture books, don't send him your fictional picture book. Figure out who publishes or agents what and cater your query accordingly.
Start your query letter with the editor's or agent's complete name, formal title, publishing house or agency name, correct address, and the date. The salutation should be formal. Pitch your book strongly in the first sentence. An attention-grabbing opening might be: Ever wonder why seals smell to high heaven? Choose something sharp, interesting and factual. Then state what genre your book is, along with your word count, in the first paragraph where you back up the contents of your one-sentence pitch. End the paragraph with a few sentences of what the story is about. Make it compelling and avoid wordiness. For a children's book, state whether it is a picture book, young adult novel, or board book.
Indicate in your second paragraph how your book topic is especially relevant, gaining in popularity and unique. Make sure your market research points are short, to the point, and relevant. If you choose to compare yourself to a famous writer, make sure your conclusions are not purely aspirational.
Show the agent that you know who she represents in the third paragraph, and let her know why you chose her. Does she represent similar genres or authors? Professional writing organizations can be helpful in this area. For example, The Society of Children's Book Writers and Illustrators (scbwi.org) has a \"members only\" section in which they provide a market survey of what editors are currently acquiring in children's books. They also have a list of agents who are willing to look at the work of SCBWI members.
Sell yourself in the fourth paragraph. Mention any unique marketing opportunities for your book. If, for instance, you are Oprah Winfrey's best friend (honestly) or you are a news anchor or you have a blog that 1,000,000 people hit every month, say so. If your marketing opportunities are not as sensational, simply discuss your qualifications as an author. For example, why exactly are you qualified to write your book? Have you been previously published? Provide titles, publisher, and publication date. Do you have any name recognition? Did you study under someone prominent who praised your work? Are you a member of a professional writing organization whose membership qualifies you to be taken seriously?
Provide complete contact information on your query letter as well as on the first page of your manuscript. Ensure there are no spelling or grammatical errors.