There's a huge range of "non-fiction book proposals," and no one guide is appropriate for all of them. For example, Susan Rabiner's otherwise-excellent Thinking Like Your Editor, which focuses on serious nonfiction, won't be much help to someone trying to put together a proposal for a collection of "true" inspirational vignettes; neither would it be all that useful for Jenna.
I've found the best middle-of-the-road general nonfiction proposal guide to be Elizabeth Lyons's, closely followed by Michael Larsen's. At the more-serious end of things, Rabiner's seems best; at the less-serious end of things, worry about having a platform rather than about the proposal. And, of course, YMMV; for example, Lyons does have a regrettable tendency to make absolute statements when she shouldn't, but that doesn't bother me because I can spot them.
And, of course, if you're dealing with a true specialty area, you must tailor your proposal to the small range of appropriate publishers. For example, there are "appearance standards" differences between publishers of works on political theory and publishers of literary criticism that will make a big difference to that book proposal on George Orwell. (Trust me, I have personal experience with that one!) The key in a specialty area is to make the proposal look as much as possible like the typical book from that publisher, including citation form, footnote usage, illustration usage, etc.
In short, there is no one true way. But you knew that already, right?
