Book proposals. Compare and contrast.
I would add the following ingredients to Nathan's recipe:
(1) Where he says "clearly differentiate your project from the other books that have already been published on the subject," that is also a good place to use your Ingram numbers to present rough sales information - order of magnitude info - for the niche you will occupy. In Kevin's case, estimate sales for books on Civil War memory and public history.
(2) Give your marketing plan. Your own marketing plan for how you will sell your book. Not the publisher's plan, your plan.
(3) You must provide a page count.
(4) Keep rewriting until the overview is irresistible.
Here's the point: your agent or editor must act as your advocate. You are equipping someone to paint a picture of achievable success and helping them to overcome objections.