This is Chuck Sambuchino's blog on how to write a great query Letter:
Think of a query as a three-part monster, broken down into three paragraphs. At the top of the page, you will have your contact info, as well as the mailing address info for the agency and the date. After that, you have your three paragraphs:
1. Explain what the work is. So - what are you writing? What is the genre? The length? The title? Is it complete? State all the basic info upfront so the agent will immediately know if this is a type of work that she represents.
2. Explain why you're contacting this agent. Did you meet them at a conference? Were they recommended by a friend? Did you see an interview online where they said they were looking for steamy romances and you're writing one such steamy romance? Show them why you picked them out of the big pile, so they have a reason to pick you out of a big pile.
1. Pitch Your Work. This is the most difficult part. You have to boil your book down to about 3-6 sentences and explain what makes the story interesting. You've got to get to the hook. What is the irony - the catch - that makes this story interesting? If your story is simply about a police officer who retires and adjusts to a new lifestyle, that has no hook. But if you say that this newly retired police officer decides to get a sex change, and finds that the police union wants to cancel his pension, and his old friends won't speak to him - then you've got a hook. You've got a unique, interesting idea for a story.
1. Explain who you are and why you're qualified to write this work. Do you have publishing credits? Are you a journalist? Have you won any awards? Have you had short stories published? If you're pitching nonfiction, this becomes the most important section of the query because you will have to prove that you are the ideal person to write this particular book.
Keep in mind that if you don't have anything to say or brag about, you can just keep this section short. Tout your accomplishments quickly and humbly. You want to say "I'm not brand new and I take writing seriously." You don't want to say "Yoo-hoo! Look at my accolades! I'm the man, if you didn't know it, sucka."
2. Thank them. Thank the agent for considering your project. Ask them if you can send more. "Can I send you the first few chapters or some pages?" "Can I send you the full book proposal?"