I'm writing a technical book, and I want to split my book up into about 3 to 6 very broad sections, each of which would have several chapters in it. Is there a standard name for these broad pieces that are bigger than a chapter? Unit? Part? Section?
What would they be called, besides "section" which seems perfectly fine?
I suggest deploying modernist systematization, especially if that would be considered egregious for your project. For example, you might model your book as a human body, with each 'part' or 'section', here termed 'organ', with the specific organ HEART or FOOT* correlating in some symbolically mystical fashion with the contents of the chapter.
*(Don't be afraid to cheat at your own weird game!)
Note that the body has been done explicitly (as part of the explicit structure) and secretly (on the part of the author) in literary fiction as well as nonfiction extensively.
Which should encourage any technical writer worried that this might be egregious.