html_chapters: Build book chapters into separate HTML files
In rameshbalan/write_book: Authoring Books and Technical Documents with R Markdown

Split the HTML output into chapters while updating relative links (e.g. links in TOC, footnotes, citations, figure/table cross-references, and so on). Functions html_book() and tufte_html_book() are simple wrapper functions of html_chapter() using a specific base output format.

html_chapters(toc = TRUE, number_sections = TRUE, fig_caption = TRUE, 
    lib_dir = "libs", template = bookdown_file("templates/default.html"), 
    pandoc_args = NULL, ..., base_format = rmarkdown::html_document, split_bib = TRUE, 
    page_builder = build_chapter, split_by = c("section+number", "section", 
        "chapter+number", "chapter", "rmd", "none"))

html_book(...)

tufte_html_book(...)

`toc, number_sections, fig_caption, lib_dir, template, pandoc_args`	See `rmarkdown::html_document`, `tufte::tufte_html`, or the documentation of the `base_format` function.
`...`	Other arguments to be passed to `base_format`. For `html_book()` and `tufte_html_book()`, `...` is passed to `html_chapters()`.
`base_format`	An output format function to be used as the base format.
`split_bib`	Whether to split the bibliography onto separate pages where the citations are actually used.
`page_builder`	A function to combine different parts of a chapter into a page (an HTML character vector). See `build_chapter` for the specification of this function.
`split_by`	How to name the HTML output files from the book: `rmd` uses the base filenames of the input Rmd files to create the HTML filenames, e.g. generate ‘chapter1.html’ for ‘chapter1.Rmd’; `none` means do not split the HTML file (the book will be a single HTML file); `chapter` means split the file by the first-level headers; `section` means the second-level headers. For `chapter` and `section`, the HTML filenames will be determined by the header ID's, e.g. the filename for the first chapter with a chapter title `# Introduction` will be ‘introduction.html’; for `chapter+number` and `section+number`, the chapter/section numbers will be prepended to the HTML filenames, e.g. ‘1-introduction.html’ and ‘2-1-literature.html’.

An R Markdown output format object to be passed to bookdown::render_book().

These functions are expected to be used in conjunction with render_book(). It is almost meaningless if they are used with rmarkdown::render(). Functions like html_document2 are designed to work with the latter.

If you want to use a different template, the template must contain three pairs of HTML comments:  and  to mark the title section of the book (this section will be placed only on the first page of the rendered book);  and  to mark the table of contents section (it will be placed on all chapter pages);  and  to mark the HTML body of the book (the HTML body will be split into separate pages for chapters). You may open the default HTML template (bookdown:::bookdown_file('templates/default.html')) to see where these comments were inserted.