Pollen: the book is a program
A book-publishing system written in Racket. Unstable. Incomplete. Et cetera. It’s becoming more civilized. But this is the actual software I use to publish & maintain my web-based book Butterick's Practical Typography.
Using Racket 6.0+, install from the command line like so:
raco pkg install pollen
And update like so:
raco pkg update pollen
Docs are installed with the package. You can also read the docs here.