A book is a business, and a book proposal is your book's business plan. Writing and publishing a book is a risky venture, with a big investment of your valuable time. Having a business plan for your book minimizes risk of loss, and increases return on your investment. Particularly for nonfiction but even for fiction, book proposals are essential for acquiring literary agent representation and sale of your manuscript to traditional publishers. Proposals are equally critical to the success as self-published books, too.