Once upon a time I received a letter from an editor, along with the rejection. If I had written the query properly, she wouldn't have had to write me the letter. I noticed at once that the publisher wasn't interested in how well-written my book was, but how well it would sell. Cynical as it may sound, we writers must realize that publishers want to know if acceptance of your book is worth the financial risk. They have to invest thousands of dollars in the production of your book, written by a nobody. Thus the query should focus on salability. Here are some tips the editor gave me in her letter.
What kind of readers will the book appeal to?
How much demand for the book do you anticipate?
What estimate can you make of the competition? Check bookstores and libraries for similar works.
Is the book user friendly? Easy to read? Not pedantic? Not too many sub-plots or unimportant characters?
Are you willing to lecture, sign books, or anything else to promote sales?
Can you provide some prominent person or expert to endorse the book?
If the book is technical, list the related organizations you belong to.
Describe the book for sales purposes.
What makes you especially qualified for writing this particular book?
If you comply with all these requirements in a one-page letter, fine, Of course, this is no guarantee that this will get you published. Think of the query as a book jacket with its teasers, only without bragging about the book.
Of course, if you know someone in the publishing business, you can forget the nature of the query letter.