I'm a big fan of the latest and greatest plugin addition to GitHub
Pages (and Jekyll), that is, Jekyll Remote Themes.
Using a simple config entry you can "automagically" pull in remote
To once more show off / test the new functionality I've updated the
Yuki & Moto Press Bookshelf  (Static Website Source)  to now
use a remote book theme (that is, Octobook Twenty Eighteen) that
turns your manuscripts in plain text w/ markdown formatting
conventions into live (online) books.
See the Gem Developer's Guide  as example.
Happy reading. Happy publishing w/ GitHub Pages (and Jekyll and Ruby). Cheers.