The most important thing to know about a book proposal is that it's not actually a proposal and it doesn't really have much to do with books.
In the publishing world, the term "book proposal" is simply a traditional nom d'art. What a book proposal really is is a business plan one creates to entice an investor (the publisher) to go into partnership with you to produce a product, which just happens to be a book.
It goes without saying (but I'll say it anyway) that publishers are in business to make money. The truth is, the publisher doesn't really care all that much about you or your idea. When a publisher considers a proposal, it's trying to figure out if there's a chance of making money after all the costs (the author's advance; the price of paper, printing, and binding; the fees paid to the designers and distributors; the retailer's slice and the cash for promotion and marketing) are added up.
How do you help the publisher make that decision? How do you convince it that there will be a nice return on investment?
You begin, on Page One, with the absolute best you got. You begin by illustrating your idea with an example and then explaining it in 400 words or less. Ideally less. Don't waste time developing or leading up to it and don't save it for a surprise ending. Remember: In most cases your proposal first will be read by a bored editor sitting in a windowless nook in a giant New York office tower. Her stomach is churning from too many cups of coffee. Also, given the parlous state of the publishing industry today, she's probably worried that she'll be out of a job in a few months and what happens then? Believe me: She doesn't have a lot of patience. If you don't grab her on Page One, she'll never get to Page Two. And note that I said "illustrate" your idea, not "tell." Nobody wants to hear somebody boast about how great their idea is; people want action; they want examples.
After you've sold the editor on the strength of your idea through the strength of your example, after you've convinced her that the whole world is pining for this book and most importantly will pay for it, you're on to Page Two in which you name the books that are like yours (only different and not as good) and already have made a lot of money. Just as Hollywood loves sequels, the publishing world likes to put the books they publish into categories and niches because the audience for them has already been established and identified and that makes marketing so much easier and cheaper.
Those are the first two pages of your proposal. What comes next?
Just as in any business plan, the investor (the publisher) needs to believe in the strength of its potential partner's management (that's you.) The publisher has to believe you're competent and can do what you say you can do: i.e., write the darn thing. The publisher also wants to know that you'll be a good partner who can help sell the book. How do you convince the editor you can do all that? That's Part 2.