def id2subdir(book_id):
    """
    Returns subdirectory that a book_id will be found in a gutenberg mirror.
    """

    str_book_id = str(book_id).zfill(2)
    all_but_last_digit = list(str_book_id[:-1])
    subdir_part = "/".join(all_but_last_digit)
    subdir = "{}/{}".format(subdir_part, book_id)

    return subdir