The query letter has one purpose, and one purpose only: to seduce the agent or editor into reading or requesting your work. The query letter is so much of a sales piece that it’s quite possible to write one without having written a word of the manuscript. All it requires is a firm grasp of your story premise.
For some writers, the query will represent a completely different way of thinking about their book—because it means thinking about one’s work as a product to be sold. It helps to have some distance from your work to see its salable qualities.
This post focuses on query letters for novels, although the same advice applies to memoirists, because both novelists and memoirists are selling a story.