def get_author_book_ratings(book_tr):
    """Get the ratings ``<span>`` element from a table ``<tr>`` element from an author page.

    Args:
        book_tr (bs4.element.Tag): ``<tr>`` book element.

    Returns:
        bs4.element.Tag: ratings ``<span>`` element.

    Examples::
        >>> for book_tr in scrape_author_books(soup):
        ...     ratings_span = get_author_book_ratings(book_tr)
        ...     print(ratings_span.contents[-1])
             4.55 avg rating — 2,414 ratings
             3.77 avg rating — 1,689 ratings
             4.28 avg rating — 892 ratings
             4.54 avg rating — 490 ratings
             ...

    """
    return book_tr.find('span', attrs={'class': 'minirating'})