"But it's surprisingly challenging to publish books on the web in nice, cohesive, tight, easy-to-navigate HTML format."

Quarto is one great option for doing that today. Bonus: it can also generate EPUBs and PDFs, all from one set of source files.

https://quarto.org/

It's free and open source.

https://github.com/quarto-dev/quarto-cli

Example books:

https://wesmckinney.com/book/

https://jjallaire.github.io/hopr/