Late 2011 iMac running macOS Sierra.
I cannot for the life of me find the documents that appear in iBooks. They are not in my iTunes folder, nor are they in ~/Library/Containers/ (in fact there is no such folder at all), nor are they in ~/Library/Mobile\ Documents/ (again there is no such folder). I do not have iCloud Drive enabled.
So where on earth are all these PDFs and epubs that I can plainly see in iBooks, can sync to my iOS devices via iTunes, etc.?