Here are a couple of great articles from Jan Friedman’s website for anyone looking to work with a publisher on a book. (This content was copied and pasted, so typos are not mine.)
Start Here: How to Write a Book Proposal
Book proposals are used to sell nonfiction books to publishers.
A book proposal argues why your book (idea) is a salable, marketable product. It acts as a business case or business plan for your book that persuades a publisher to make an investment. Instead of writing the entire book, then trying to interest an editor or agent (which is how it works with novels), you write the proposal first. If a publisher is convinced by your argument, it contracts you and pay you to write the book.
If properly developed and researched, a proposal can take weeks or longer to write. While proposal length varies tremendously, most are somewhere around 10 to 25 pages double-spaced, not including sample chapters. It’s not out of the question for a proposal to reach 50 pages or more for complex projects once sample materials are included.
New writers might find it easier to simply write the book first, then prepare a proposal—which isn’t a bad idea in the case of narrative nonfiction, since many editors and agents want assurance that an unknown writer has sufficient writing chops to pull off their project. But having the manuscript complete does not get you off the hook when it comes to writing the proposal. (read more)
And if you’re still mixed up about defining your readership, try this article:
How to Define and Describe Your Readership: A Confusing Issue for Nonfiction Book Proposals
If you’re pitching a nonfiction book, at some point, an editor or agent will expect you to describe the readership that your book is intended for. Or, if you’re self-publishing, you’ll need to define this for yourself to market the book properly.
Sometimes a book’s readership seems obvious from the title alone. For example, we can reasonably assume that Running Your First Marathon is for people who are training for their first marathon. Easy, right? Yet consider that such a book might be written for a more advanced or serious runner than, say, The Non-Runner’s Marathon Trainer, which indicates a reader who has little or no history of running. The title tells part of the story—but not the whole story.
Thus, even books that strongly indicate their readership in the title require useful elaboration from the author on who the book is meant for—or perhaps who it is not meant for. (read more)