SELECT
    title,
    array_remove(array_agg(
        CASE WHEN type=ANY(ARRAY['author', 'poet', 'translator']) THEN fullname) ELSE NULL END
        ORDER BY array_position(ARRAY[
        'author', 'poet', 'translator'
        ], type)
    ), NULL) as authors
FROM books
GROUP BY title
ORDER BY authors ASC;