"The main objective of a query is simple: Make the agent care enough about your protagonist and your plot that she wants to read more." (Mary Kole)
The query is your first, and maybe your only, chance to get the attention of an agent or publisher. Make it count. Remember your book is a marketable commodity. Make sure your manuscript is polished and ready to go before you submit your query letter. Author Jane Friedman offers some excellent suggestions for anyone drafting a query for a novel (http://janefriedman.com/2014/04/11/query-letters/).
She recommends that you mention how you came upon the agency or publisher you are querying. For instance, you may know another client of the agent or another author of the publisher. Mention that in your first paragraph. Perhaps you heard him or her speak at a conference. Include this information as well.
Include any impressive credentials you may hold; for example, maybe you have a MFA from a university from which the agent is known to recruit authors. Perhaps you won a national writing competition or you have written for prestigious journals or New York publishers.
Ms. Friedman says that good salespeople develop a rapport with customers and understand their needs. You have to do the same with your agent or publisher. Make sure you personalize your letter. Include information that sets it apart from the hundreds of other queries sitting in the slush pile on a publisher's desk.
Your hook is what grabs the reader's attention from the get go. Make sure you include your protagonist's name and what conflict he or she is involved in. List the choices the protagonist has made in response to this conflict. Ms. Friedman says the hook should have "sizzle". Give examples of how your novel is filled with "strong actions, strong ramifications and and lots of emotion tied to each." Agents eat this stuff up, according to Mary Kole (http://www.writersdigest.com/online-editor/how-to-write-the-perfect-query-letter). You might also mention the setting or time period, especially if it is historical fiction.
Your hook needs a certain "je ne sais quoi" says Jane Friedman. It should not seem flat: inject your writing with "life, voice and personality". However, at the same time, don't ramble on. Brevity is also key: limit your explanation to under 200 words. The more you explain, the more you might "squeeze the life out of the story," says Ms. Friedman.
Refrain from revealing the ending of your story. Reserve that for the synopsis. Mention no more than three or four characters. Avoid mentioning minor plot points. Avoid editorializing your story.
Here is a successful query for a mystery novel by Robert K. Lewis called Untold Damage:
"Mark Mallen has been a great cop before he succumbed to the needle. Driven from the narcotics division and run off the force Mallen's been surviving day to the day in the gritty world of San Francisco's Tenderloin district. But just as if it looks like his life will end in addiction, his upstairs neighbour comes to him begging for help in finding her daughter's murderer. The man hidden deep inside the addict is prompted to action as Mallen sees an opportunity to redeem himself and re-emerge."
After writing your hook and a some plot points, it's time to write your bio. Make it meaningful and charming. Again, it should stand out to the agent or publisher who is reading it.
Next come your writing credentials. Make sure they are specific; for example, if you have written for magazine, mention their titles. If you have no fiction experience, include non-fiction credits. It shows you have worked with an editor. Popular online journals and blogs also count. Only mention self-published books if they are relevant or if you have sold an incredible number of copies. Include your line of work if it relates to your story; for example, if you are a teacher writing a children's book, that's useful information to share. List your writing degrees? Mention if you belong to any professional writing organizations (ex. RWA, MWA, SCBWI) or attended any major writer's retreats or workshops. You can include your website or blog, especially if you have 100,000 plus followers.
Here is an example of a bio included in a successful query letter for the novel Finny by Justin Kramon:
"Let me tell you a bit about myself. I'm 27 years old, a 2004 graduate of the Workshop with an MFA in fiction. The collection I've finished was awarded the Michener-Copernicus Society of America Award and I've received several other fellowship awards including a Sun Valley Writers Fellowship and a Book Hampton Fellowship. I've published about half the stories in my collection in literary magazines including Glimmer, Train, Fence, Story Quarterly, and Boulevard. A story published last year ("Shel" in Glimmer Train) was selected by the most recent Best American Short Stories as one of their "100 Distinguished Stories". (http://www.writersdigest.com/editor-blogs/guide-to-literary-agents/successful-queries-agent-ayesha-pande-and-justin-kramons-finny)
Include any special research you conducted for your book; for example, you spent a year as a missionary in the Congo; you lived in a commune in Israel; you climbed Mount Kilimanjaro (like Eric Walters did for his middle grade novel Between Heaven and Earth).
Jane Friedman reminds you to limit your query letter to one page. The standard debut novel length is 80,000 words. Avoid bragging about your work. At the same time, avoid criticizing your story. Do not overuse adjectives and adverbs. Let your query speak for itself. Last, but not least, don't forget to thank the agent or publisher for his or her time!
Note: To read several successful query letters, visit http://www.adweek.com/galleycat/23-literary-agent-query-letters-that-worked/77310.
For more information, read Query Letters That Rock by Linda Formichelli.