Books

https://github.com/seandheath/koboctl/blob/main/internal%2Fhardening%2Fhosts.go#L20

the code at the very least works, and blocks trackers including google analytics, google ad tracking and hotjar behavior analytics. presumably this would happen in the default OS and initiated when clicking on (and maybe even turning pages on) any book, including sideloaded ones. I assume this doesn't work in koreader regardless of whether you disable those domains or not. If you want you can always contact the author.