Disciplined Entrepreneurship by Bill Aulet. I won’t repeat or even summarize what he wrote. If you haven’t yet launched or achieved more than $1 million of revenue, go read Bill’s book first.

Having no clear, written goals for your startup means you won’t know whether to pursue the white label deal someone offers you two weeks after launch, or to start selling in overseas markets because someone asks you to.

Try to answer every possible question that that person could raise. That's the description of a business plan, really.