Check out https://typst.app/ if you're looking to write a book yourself.
https://hypermedia.systems/ was written with it.
https://hypermedia.systems/ was written with it.
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I'm a bit confused; the site claims HTML and PDF outputs are "Coming soon"
what export formats are supported? HTML and PDF seem very significant to me, and are how I'd likely share most documents with non-tech inclined people.
As of 0.4.0, Typst supports PDF, SVG and PNG outputs. The Typst team seem to be rapidly adding features, but I don't think HTML support is coming soon.