A query letter is a business letter. The purpose is two-fold
Entice the agent to read your pages/request the full manuscript
Demonstrate you are not an asshat.
QueryShark is a book agent. (Book agents call themselves literary agents, but so do screenwriters' agents, so there you go: book agent.) She critiques query letters on her blog.
Queries for novels are allowed to be longer than screenplay queries, which, I keep telling you, ought to be ridiculously short. However, many of the same do's and don't apply. Throw the hook out there, sell the sizzle, stop talking while they still want to read your goddamn manuscript.
Check out this excellent post
, which boils down a seminar she gives into bullet points.