There is an interface to get the list of files that are selected in BookBrowser.
So if you only want the current file or a specific subset of files, make sure they are properly selected in BookBrowser before launching your plugin. The interface call is here:
https://github.com/Sigil-Ebook/Sigil...tainer.py#L313
Alternatively your plugin can use either Tk or Qt to allow the user to select any set of files they desire.