This is the book proposal that got me a contract to write my sixth book, about public speaking.
Obviously, all book proposals are different, and this is just one specimen. Yours will have a different subject, and tone.
But this will give you a sense of what to include.
It lacks a cover page, and contact details for my agent - possibly other things too, but this is essentially what was submitted.
After sharing this document on writing courses at the Arvon Foundation and elsewhere, I thought that others (you?) might find it useful.