If the book contains parts, the div.part elements should become a new parent div that contains all chapters until the next part or until the first appendix section.
Added handling to replicate current part handling (wrap part in div, but don't wrap the div around the child sections). We'll need to expand this when we're ready, which will also entail updating all CSS.
If the book contains parts, the div.part elements should become a new parent div that contains all chapters until the next part or until the first appendix section.