I think (as most here seem to think)that query letters need to convey the book. I also think you should demo your professionalism, and your willingness to work with editors. This is not a real query. I am just making it up, but it's the general pattern I prefer to use. I often get to pitch live now and I usually follow this progression with my mouth as well:
Story hook, statement of credentials, tiny demo of market savvy, mention of editorial input being welcome and an inference of productivity.
Just like a synopsis the task is to say everything in a few words. Just like a job application, your general work habits are of interest, past experience and special areas of expertise that impact the book being pitched should be included.
Dear, Nameof Editor, My middle grade novel (brilliant title here) starts with a girl in tears. Sarah is facing foster care--becoming a ward of the court. There is nothing she can do to help her dying mother. And there is no way to trace her father. It's up to her to think of something, or she is going to lose her little brother, too. The social worker was honest: they will not be kept together in the state system. Sarah is only 12, but she knows how to drive. She had to learn, to help her mother get back and forth to medical appointments. And she knows where the box of cash is--her grandmother's last gift. At midnight, holding a pin and staring at a map from the gas station, she closes her eyes, circles her hand and jabs downward. Mobeetie Texas. Why not? She stands up and shivers into her jacket, folds the map, then goes to wake Tommy. ((continue the storyline without excessive detail and give the editor a clear feeling of how it ends, physically and emotionally, for the protagonist. ))
Middle section: State your writing credentials briefly. If, as in this case, you have worked with kids in shelters, put it in. If you have worked with dogs in shelters, leave it out. Points for pertinence.
End section:(Brilliant title) will appeal to the audience that loved (name a few books that have done well or won biggie prizes that you think appeal to the the same age group, appeal-type/category as your work), The book is complete, has had outside readers, and has been preliminarily revised. I look forward to working with editorial suggestions.
Sign off sentence, reminder of hook plus bonus info:
Sarah and her brother will be waiting for your response. I will be working on my next novel, also for middle grade readers. Thank you so much for your time. sig.