Store plugins can do only one thing, provide a list of results after searching a website. You want to do all the other things, you need to create a user interface plugin.
And as for APIs to add books to calibre and read metadata, they are present and can be used from user interface plugins.
Simply use one of the methods from self.gui.iactions['Add Books'] (which is the Add Books builtin user interface plugin) to add a file to calibre, reading metadata etc is handled automatically.
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