So, really quickly, what is a book proposal? A book proposal is essentially the sales plan for your book – it serves to answer the questions of agents and editors before they ask them. It tells them what your book is about, why you’re the one to write it, and who is actually going to go out and purchase this book. Publishers want to invest in books, but they want to invest in authors, too, and they want to know that they will be making back their investment. (And agents want to know that the clients they are pitching are actually going to sell.)