A book proposal makes the case for the viability and marketability of your nonfiction book with a traditional/legacy publisher. It is also crucial as you look for representation (literary agent, etc.) if you decide to pursue traditional publication.
In effect, the proposal is a business plan for your book, one that persuades a publisher to make an investment. So, instead of writing the entire book up front, then trying to interest an editor or agent (as with fiction/novels), you write a proposal first. If a publisher is convinced by your argument, it contracts you and pays you an advance to write the book.
A solid proposal will be a document approximately 35-50 pages in length. It includes research about the potential market for your book, as well as a substantial writing sample.
CMP can help you with this. If you have a book idea, or have already started your nonfiction project, contact us for details.