If you are looking to secure a literary agent, you might be interested in this query letter advice given by literary agent Alice Speilburg at an impromptu gathering attended by author Celeste Bennett in 2014.
1. Query at last 15 agents. Every agent knows you are seeking representation and knows you're going to submit to more than just them. If you get an agent that shows interest you might be able to leverage others to take a closer look at your work.
2. Most agents don't routinely respond to a query unless they are interested. If you haven't heard back in 5-6 weeks e-mail check in with the agent again just to ensure they got your query. It might prompt them to pull it out of the slush pile and look at it again. In any case, you'll know if it wasn't received or wasn't well received.
3. Don't waste an agent's time with a query letter until you've got your fiction novel done. Non-fiction requires a book proposal submission.
4. Follow the agent's guidelines for query letters and submissions. It's a simple kick out for your query if you don't include everything the agent stated they needed to evaluate your work.
5. You don't need to mention every single character or reveal the story's ending in your query. Give enough detail so the agent knows the novel is complete and interesting, but not so much it makes the query letter too lengthy or confusing.
6. Try to let your writing style shine through in your query letter. It's also great to include a 'this meets that' where you take two popular, unrelated fictions/styles and show how your work fits into both. It can be a movie, t.v. shows, books, etc. Anything that gives the agent a point of reference for understanding your work.