While the webpage structures are mutable to your needs, the print.html and layouts is special.
In it, a fixed number of pages are generated (you can change this number by editing _config.yml). There’s a lot of nesting to keep track of details, and this is the list from outside in:
.sheet, .page, .content-wrapper, then either one or two .page-content (which can help facilitate a two-column layout), then .content divs that have content flowed into them.
All of the content comes from book.html.