def num_of_books_complete_message(tracker):
    """Return the number of books complete in a message string format."""
    num_of_books_complete = tracker.shelf.num_complete_books
    message = f"You have completed {num_of_books_complete} book"
    message += "s" if num_of_books_complete != 1 else ""  # Add plural
    message += " so far."
    return message