You have a great idea for a nonfiction book, but don't have a clue as to how to put together a proposal.
Proposals are mandatory.
Most of the larger publishing houses will only look at a proposal. This is less time consuming and it gives them the opportunity to review the market, the competition, and your promotional ideas. Yes, it is your responsibility to analyze the market, ferret out the competition, and create promotional ideas. You are not only the writer but the idea broker. A book proposal is a sales tool.