If you're just joining us, see How To Write A Book Proposal, Part 1. Even though this post should be called, "How To Write A Book Proposal, Part 2," I changed it to, "Why A Book Proposal Is Everything," because ... well, because "why?" most naturally comes before "how"? Sorry for not thinking of that sooner.
There are three Major Reasons for which you have to write and submit to your literary agent or publisher a book proposal instead of a finished manuscript. (And remember, we're only talking about nonfiction books here, not fiction.) First, publishers don't have time to read a 40,000-plus word manuscript. They don't even have time to read anywhere near all the proposals that every agent in the world is sending them. (Which is why, as you climb up the publishing ladder, you want representing you an agent with whom publishers know, respect, and have previously worked, since a submission from such an agent automatically goes atop publishers' Must Read stack.)
Proposal? 15,000 words. Whole manuscript? 45,000 words. Publishers' time? Priceless.
A proposal it is, then.
Secondly, the quality of your book idea and the facility with which you write is one thing. But what really matters to a publisher -- who after all has to make a living selling books -- is how sellable your book is. Before a publisher commits the kind of money it takes to bring a book to market, it has to be as sure as it possibly can be that that book will sell. Determining that -- figuring out how many people can reasonably be expected to buy your book, and why -- entails considerable thought. That's where you come in. That's largely what a proposal is: It's your summation of all the reasons the publisher reading it can be safe betting that once your book is published the world will flock to it, and he or she will be rich and get a promotion and get to take the spouse and kids to Paris the following spring.
A proposal is a sales document. It's a pitch. It's everything an editor would need to know in order to boldly throw your proposal down on the table before the collected editorial, sales, and marketing people at his publishing house, and say with ringing confidence, "Here. I've got a winner. Praise me, ye underlings! Marvel yet again at my awesome perspicacity!"
Or, you know, whatever they might say.
Point is: Books are art. Art isn't quantifiable. Money is. Publishers want to make money. A proposal is your best effort to show publishers that, artistic wonder or not, your book will result in Mucho Incoming Cash.
Thirdly, publishers don't want you to have already finished your book before they get it. You know why? Because if there's one thing of which publishers are confident, it's that they know what makes for a good, sellable book. They want to participate with you in the writing of your book. They want to help you make it the best book it can be.
You are, after all, just a writer. What in the world can you be expected to know about writing a book?
It's easy enough to be offended and/or disparaging about the degree to which publishers tend to assume a real kind of ownership of the text of the books they publish. And a lot of what they do in that regard is grounded in nothing more interesting than grunt arrogance: Editors and publishers are, after all, the gatekeepers to fame and fortune, and they know it, and ... well, you know how people are. But it's also more than fair to say that through long and hard experience, editors and publishers have learned that the most efficient way to create the best possible books is by working hand-in-hand with their authors. Especially given that most nonfiction authors aren't primarily writers; they're primarily experts in whatever it is they're writing about. Most often nonfiction authors are glad to benefit from the knowledge and expertise of their editor; they understand the value of that kind of input. So it's all good. It's just that if you're new, you want to know, going in, that you'd do well to hold lightly the sense of proprietorship that most authors naturally feel toward their work. It's your book until you sell it; after that, it belongs to you and the publisher, and no two ways about it.
Tell me how/why this could possibly interest you in the least here.