The protocol for submitting fiction to a literary agent or publisher differs from submitting nonfiction. Typically for fiction, and most certainly for unknown fiction writers, agents and/or publishers want to see a completed manuscript. But for nonfiction, they usually request just the first two or three chapters of a manuscript-in-process, plus a proposal.
Might sound like nonfiction writers are getting a break, but they are not. A well-constructed proposal, the nonfiction-writer’s primary marketing tool, is a huge task. Not only does a proposal require focused, polished writing, but also the type of writing, the genre, if you will, differs drastically from the proposed writing.
A book proposal is a sales pitch written to persuade a businessperson (agent and/or publisher) that readers (your ultimate audience) will buy your writing. And most writers simply want to tell a story, not sell their story or themselves.