Ask HN: Advice / resources for writing a nonfiction book proposal?

I'm a web developer-by-way-of-journalism writing a book that uses data to tackle issues in popular culture (very broadly). I've gotten a fair number of responses to my query letter from literary agents interested in a formal proposal, but I don't really have any resources for putting one together (other than what I've googled) as my background isn't in publishing.

Does anyone have advice for nonfiction success? Or could recommend a service for reviewing / critiquing it? Or might even be a publishing insider up for the easiest consulting gig ever?

Thanks HN!