Pollen [1] is pretty neat, it's amazing what Matthew Butterick has done with it to create his books. Beautiful Racket [2] truly is beautiful and inspiring.
You're not kidding, that's probably the most beautiful book in HTML I've ever seen. The typography is gorgeous, the navigation is intuitive and easy to use, downloadable code samples, links for every paragraph with a feedback form (!), and the writing style is clear and approachable.
It's definitely on my to-read list. Thanks for sharing!
[1] https://docs.racket-lang.org/pollen/
[2]: https://beautifulracket.com/