def format_reminder_email(quest_id: str,
                          days_signed_out: int,
                          librarian_name: str,
                          book_name) -> str:
    """
    Formats an email as a plain string for sending out email reminders
    for signed out books.

    Example: _send_email("s455wang", "2017-02-04", 30, "csfmurph", "How to Design Programs")
    """

    email_message_body = \
"""Hi {},

Our records indicate that you have had the book {} signed out for {} days.

Please return the book to the CS Club office (MC 3036) at your earliest convenience.

Thanks,

{}
Computer Science Club | University of Waterloo
librarian@csclub.uwaterloo.ca""".format(
        quest_id,
        book_name,
        days_signed_out,
        librarian_name
    )

    email_message_subject = "Overdue book: {}".format(book_name)
    email_message = "Subject: {}\n\n{}".format(email_message_subject,
                                               email_message_body)
    return email_message