As @Nad wrote in T124640#1961966 and after, people going to https://www.mediawiki.org/wiki/Extension:PdfBook might either click the Github link and get one codebase, or click the "Download snapshot" link and get another codebase (on Wikimedia Git).
It's probably in everybody's interested to not have two separate codebases around when there was no specific reason or intention (like disagreement among maintainers) to fork, but instead to point / redirect contributors to a single place if possible.
@Nad:
* If you are interested in taking ownership of the Git repository, please [[ https://www.mediawiki.org/wiki/Developer_access | get developer access ]] and check https://www.mediawiki.org/wiki/Gerrit/Project_ownership#Requesting_repository_ownership (note that currently [[ https://www.mediawiki.org/wiki/Gerrit | Gerrit ]] is used for code review which will get replaced by Phabricator's [[ https://secure.phabricator.com/book/phabricator/article/differential/ | Differential ]]).
* If you prefer to stay on GitHub, please let us know so we can mark the Wikimedia Git repository at https://phabricator.wikimedia.org/diffusion/EPBO/ as inactive and hopefully/somehow also correct the download link on https://www.mediawiki.org/wiki/Extension:PdfBook