I am happy to see properly filled @copyright tags in the plugin code, thanks for that. I noticed that in some files - such as https://github.com/RichardPilbery/moodle-booktool_epubexport/blob/master/version.php#L21 there is only the original author left.

In cases like this - typically version.php which has no actual code you reused - I would recommend to leave your name only. Or, if you prefer, always make sure to add yourself there. When it is like this, people might be confused expecting that the plugin was actually written by Petr. Thanks.