IDEAs

Also generate a PDF book

I could use pandoc for this (convert from Markdown to PDF). This works on Fedora Linux 34:

sudo dnf install pandoc wkhtmltopdf
pandoc **/*.md --pdf-engine=wkhtmltopdf --output foo.zone.pdf

There will be some more scripting required to get the page order and ToC correct.

Use GNU source-highlight for code highlighting in bare-blocks. Can add lang in block beginning. See also https://www.gnu.org/savannah-checkouts/gnu/src-highlite/style_examples.html
Automatic ToC generation.
Sitemap generation.
More output formats. Gopher? Groff? Plain text? PDF via Pandoc? .sh with interactive menus?