Function layout_book 

pub fn layout_book(
    book: &Book,
    styles: &StyleTree,
    registry: &FontRegistry,
    assets: &Assets,
) -> LayoutOutput

One book through the whole pipeline: lines laid out, flowed into pages, everything the output needs assembled.

A single run over a session that retains nothing. It keeps one section’s lines at a time, which is what a process that renders a book once and exits wants. A live preview uses Session instead.

assets is the images the host probed. A book with none of them passes Assets::none.