This looks nice! I would love to have an ebook version to read on my ereader. I wonder how much effort it would take to use the markdown files in the GH repo and convert those.
You can clone the repo and install pandoc. Then run "pandoc index.md *.md -o operating-system-in-1000-lines.epub" in "website/en/" folder and you will have a fully working ebook.
[0] https://github.com/nuta/operating-system-in-1000-lines