Query Common Sense
The agent/publisher query is the most important business communication a writer will make. It has to be succinct while creating a desire for more. It must not be cute or rely on a gimmick. It must be confident, but not over-confident. It must display the plot while evincing there is a business case for the book. It must be free of errors. It must be honest and authentically you.
It must be many things and do it all in as few words as possible.
And, there is no formula or recipe for success. Anyone who tells you that if you buy their e-book it will elucidate the true secret of publishing success is a lying bullshit artist. You could write a great query and still get rejected by ninety-nine percent of the agents and publishers you reach out to, but all you need is the one percent, the one yes, to get the ball rolling.
You may also write a great query and fail one-hundred percent.
But there are things you can do to increase the odds you will get noticed (in a good way) and asked to submit a proposal and samples or the entire manuscript. They are:
Unless the agency has a single query@... email address, use the agent's name in a businesslike way. Mr. Smith... Ms. Smith... (Ms. is neutral as to marital status and something the world should aspire to as a default).
If they have the generic query@... address, then simply say Hello,... and in the next line, My name is ... and I am writing to see if you might be interested in reviewing my ... word, creative nonfiction/fiction/romance/etc. manuscript.
Include a bit of detail that shows you did some research into the agent's likes and dislikes. This doesn't have to be long, but demonstrates there is a reason you chose to send them your very important business communication.
Get right into the summary, which should be compelling and very brief. Two paragraphs at best, one if you can pull it off.
Since most good writing is character driven, include a couple well-known characters your protagonist resembles, but be careful not to oversell.
Give an example of that character's voice with a few great lines from the manuscript.
If it's a true story let them know how close it is to actual events by briefly describing your research. They will learn this more fully in the proposal, so be brief.
Do you have a platform or unique sales angle? If so include that you write a popular blog or have a way to speak directly to the market for your book. Be brief, just enough to titillate.
Tell them if it is complete and edited as well as who edited it (hopefully a pro).
Tell them if you have a proposal that lays out a fuller business case for the book.
If their guidelines request samples or anything else tell them it is attached (if they accept attachments) or pasted below your signature (most prefer pasted into the body of the email).
Thank them for taking the time to read your query and close with sincerely, or best, and your name and contact info.
In all, this should be a page or less, preferably less.