When it comes to structure and content, I've heard a number of authors fret over "getting it right," that interesting notion that there is a right way and a wrong way to decide upon the form and content of a book, and they are much concerned about discerning the right way. One woman asked me "When you set up chapters, do you have to do x?" as though there's a rule for this, and that I could quote it for her. A man recently asked me to help him know "what editors expect" when it comes to scope and content, as though there is some sort of quantifiable concensus among editors on anything, ever.
Books are in one sense physical utensils, and their apparatuses have developed over time in response to readers' collective needs. Chapters are simply a further organizational tool -- a way to make sense of a long train of words in readable units. Books, sections, chapters, headings, etc. exist to serve us, not we them. Content and scope are responses to format... you can't go on forever in one book; you have to stop somewhere. By asking what the material requires in order for it to be best comprehended by busy, distracted readers with many other choices, we'll get a lot closer to helpful answers than by worrying about what's "right," or what an imaginary group of conferring and agreeing editors might have decided in their Uniform Code of Scholarly Desirability.
These imaginary publisher expectations contribute greatly to authorial anxiety, and existing examples in book history can act as calming guides. Books that have pleased you personally and also prevailed in a crowded marketplace can yield satisfying and stress-reducing structural templates. I'm not saying you should copy them, but that such models frequently offer workable inspiration. And what a joy it is to sit in the cafe of a big bookstore with a stack of potential literary models, mapping them on paper and fruitfully whiling away a glorious afternoon.