After a book is rendered, there will be a series of output files and
directories created in the book root directory, typically including
_book/, and some HTML/LaTeX
auxiliary files. These filenames depend on the book configurations. This
function identifies these files and directories, and delete them if desired,
so you can rebuild the book with a clean source.
clean_book(clean = getOption("bookdown.clean_book", FALSE))
Whether to delete the possible output files. If