Version 1.1 released in post #1. Only a minor fix to show a sensible error message if no books are selected.
@craftydan, revfish, viviena - Thank you for testing
@craftydan,
Thank you for the feedback. I have updated the instructions as per your suggestions and implemented the 'no book' fix. Making the Customization 'GUI pretty' is another matter, though. At the moment I am limited to a simple text string in Calibre GUI plugins. There may be an advanced Calibre plugin feature which would allow a full customization bells & whistles screen but if so, it's beyond my current skills. If enough people are interested in the plugin I may try to pursue the matter.