Best way to publish a free book online?

See if sphinx-doc[1] suits your needs. http://d2l.ai is a good example of an online book done using Sphinx. Supports discussions through Discourse integration.

You write content using reST or Markdown (HTML is also possible). Sphinx generates a static site. JavaScript can be used for dynamic content integration. Supports built-in keyword search using only JS. A theme like mxtheme makes it look good too.

Another option is Jekyll. Similar thing, except that Sphinx requires Python while Jekyll requires Ruby. https://course.fast.ai is a good example.

[1] : https://www.sphinx-doc.org/en/master/

There is a fork of gitbook honkit discussed here https://news.ycombinator.com/item?id=23659451

I ran across this person's blog, who seems like they have some different slants on publishing that might interesting to you:

https://www.jeffgeerling.com/blog/6-lessons-learned-self-pub...

Maybe just make a site for your book using Hugo (or Jekyll/Pelican/etc).

If you expect high traffic volumes, put it behind a CDN such as Cloudfront.

Offering multiple download options such as EPUB and PDF might entice more readers.

mdBook [1] is an option if you want to convert markdown to a static website. There's plenty of customization options but I don't know if it is possible or easy enough to add comments.

Or just publish it on GitHub? There's a discussions feature but I don't think it is out of beta yet. Until then you could use issues for discussions. Or reddit may be?

[1] https://github.com/rust-lang/mdBook

Have you looked into the Amazon Kindle Self-Publishing option?[1]

[1]https://kdp.amazon.com/en_US/

I suggest you to try gumroad.com I have used it in the past.