[capink]

You can try the following code (copy/past into the module editor):

from calibre.ebooks.metadata.meta import metadata_from_formats
from calibre_plugins.action_chains.actions.base import ChainAction

class CoverFromBook(ChainAction):

    name = 'Cover from book'
    support_scopes = True

    def run(self, gui, settings, chain):
        db = gui.current_db.new_api
        book_ids = chain.scope().get_book_ids()
        for book_id in book_ids:
            fmts = db.formats(book_id, verify_formats=False)
            paths = list(filter(None, [db.format_abspath(book_id, fmt) for fmt in fmts]))
            mi = metadata_from_formats(paths)
            if mi.cover_data:
                cdata = mi.cover_data[-1]
            if cdata is not None:
                db.set_cover({book_id: cdata})