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As @ChrisSunami says, most non-fiction books are sold before they are written. Typically you have a synopsis, the Chapters and an outline for each, and for some books, a list of people you plan to...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47610 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
As @ChrisSunami says, most non-fiction books are sold before they are written. Typically you have a synopsis, the Chapters and an outline for each, and for some books, a list of people you plan to interview (along with commitments from them). You will also need a query letter to agents or publishers, and a sample of your writing for the chapters, like write the first chapter. Some people will write their book first and try to sell it; just like fiction a finished work is easier for an agent to sell. Particularly if you have no compelling credentials or publication history. Most non-fiction writers have some sort of authority on their subject. For example best-selling novelists write books on writing, or getting an agent, or self-publishing. Unlike breaking into the fiction market, publishers tend to want that subject-matter authority to sell your book. For example, you probably won't sell a book (to a publisher) explaining quantum mechanics without a doctorate in fundamental physics.