A cover letter is usually sent to publishers and magazines to go along with your manuscript.
A query letter is sent to agents in order to reel them in and wet their appetite. It is a pitch that is destined to get the attention of an agent who will represent your work and present it to publishers.
What to include:
1) Never use a rhetorical question. Agents want the writer to tell them, grab them, be professional. They will stop reading after the first question because a question does not say anything about the content of the book. What is the point of asking a question nobody wants an answer to? Isn’t that a waste of space?
Nathan Bransford, agent with Curtis Brown, says, “There is no way to make a rhetorical question an interesting start to a query. […] The more extreme the rhetorical question (Have you ever wondered if space aliens live in your underwear drawer?) the more I want to say, "NO." The more mundane the rhetorical question (Have you ever felt sad?) the more I want to say, "NO." Only the second no is more of a sarcastic no, like noooooo, rather than the first no, which is more of a serious no. It's all about inflection, people.”
2) Keep your sentences short and the query under 300 words. Do not tell the whole story. This is not a synopsis. Just tell enough to wet the agent’s appetite. Stick to the main plot (do not mention the subplots). Test your query on someone and see their reactions, the questions they ask or if they are intrigued and want to know more.
Be specific. Do not use generalizations. And include the plot. Focus more on the emotional impact on the character than the action itself. Introduce the big question of the novel. Include the decision that will change the life of the character.
Example: What is Peter Pan about in a few sentence? Peter Pan is about a mischievous boy (character) who refuses to grow up (plot) and steals away kids to make up his gang of lost children (incident). Peter meets Wendy and takes her to Neverland (decision that will change Peter’s life). There, the little girl is torn between the childish fun and the potential of a grown up life and growth (first conflict). Under Wendy’s influence, Peter’s gang will sway between growing up and being adopted or keeping their innocence and remaining with Peter (second conflict).
3) Use correct grammar and spelling. Check the meaning of the words you use. An unprofessional query letter tells a lot about the ability of a writer.
4) Work on the tone of your query. If your novel is humoristic, the query should reflect that. Actually, a query should reflect the personality of the writer and the special tone of his novel. However, do not write as if your character was talking.
5) Follow the submission guidelines. Agents and publishers post them for a reason and they are very particular about them. Each agent has his own preferences and it can be very different from one agent to the other.
And make sure your novel is finished (when you cannot add or remove one more word) and professionally edited.
6) Learn about the agent you are contacting. Check the novels she has successfully agented. Learn about her tastes, the genre she is representing and find out why you want her to represent you through her interviews or blog.
And watch out for the bad agents and the quacks; there are plenty out there.
7) Some agents appreciate it if you mention your publication credits, writing experience, activities like appearances, and education that is relevant to writing your novel. Mention where you got the expertise in the subject you are dealing with in the novel.
Example: If you are talking about ghosts in your story, mention that you are a member of the local ghost busters. If your main character is an archeologist, mention that you worked on excavating sites.
However, most agents do not want to know that unless you are already a published author.
“You do not need credentials or "relevant life experience" with a fiction query. Many respected agents have said as much, though if you need me to find specific links I can go hunting through my bookmarks. With fiction the only concern is whether the queried book is a good fit for the agent. If the writing is compelling, that's all the credentials and experience needed -- and they'll be looking at your query to determine the quality of your writing before they look at pages, usually.” K.L. Morgan
Nothing here if it is an email.
Dear Mr./Ms. Last name (check spelling):
2 sentences (or more if you know the market well): I picked you because… or novel genre and word count. Your knowledge of the market (I read your interview in … You mentioned… The market for this is…). Compare to another book the agent represents.
1 paragraph: Character (2 maximum). Setting. Plot (emotional drive). Decision that will change the MC’s life. What will prevent the MC from getting what he wants versus what he needs: conflict. Villain. What is at stake (the catastrophy ready to happen).
1 paragraph: specifics about the novel that makes it unique.
1 sentence: Title of novel is my debut novel. It is a WORD COUNT –word GENRE written for TYPE OF READER.
2 sentences: writer’s bio and how he will be marketing this novel. Any platform goes in here like website, how many followers on twitter, Facebook if consequential.
1 sentence: "Thank you for your time and consideration."
“15 Resources for a Better Query Letter” by Suzannah Windsor Freeman
25 Reasons Your Query Letter Sucks by Suzannah Windsor Freeman
How To Write A Query Letter
“Successful Query Letters for Literary Agents” by Jason Boog.
“Anatomy of a query letter” by Cynthea
The Query Shark
“How to Write a Bio Paragraph in Your Query Letter” by Courtney Miller-Callihan