Book proposals may include sample chapters, a market analysis showing why the book would sell well, and references to specific qualifications of the author that make them suitable to write the book.
Book proposals are most often used with non-fiction books, because of additional costs associated with writing the book, such as travel and research, which the author is not willing to pay without knowing the book will be published.
In independent publishing, book proposals are unnecessary. However, parallels to the book proposal exist, such as an author's pitch for a crowdfunding campaign, a call for patronage, or a list of benefits to subscribers in a subscription model.
In every case, the objective is the same: to persuade someone other than the author to invest or share in the costs of producing the book.