For every piece of content you put in your book, you need to know the answer to the question “why is this here?” And the answer has to point back to the book’s (& chapter’s) thesis. Answer can’t just be “I found this other thing when I was researching & it’s super interesting.”

For the purposes of yr book proposal, u can come right out & directly answer the question “why is this here” for every piece of content u discuss (e.g. in your annotated table of contents). U need to convince yr reader that yr plan for the bk makes sense, even if not written yet

You don’t have to say “this is here because ________” because that would get boring and repetitive. But you can always tie back in to your argument, which will essentially communicate why you’ve decided to put whatever info/discussion/analysis in there.