Подскажите почему происходит вывод

Подскажите почему происходит вывод на экран не адреса ссылки на объект, а [Tom Sawyer was written by Mark Twain, Agatha Christie: Hercule Poirot is a detective]

public static void main(String[] args) {
        List<Book> books = new LinkedList<Book>();
        books.add(new MarkTwainBook("Tom Sawyer"));
        books.add(new AgathaChristieBook("Hercule Poirot"));
        System.out.println(books);
    }

...

        private String getOutputByBookType() {
            String agathaChristieOutput = author + ": " + getBook().getTitle() + " is a detective";
            String markTwainOutput = getBook().getTitle() + " was written by " + author;

            String output = "output";

            if(this instanceof MarkTwainBook)
            return markTwainOutput;
            if(this instanceof AgathaChristieBook)
            return agathaChristieOutput;

            return output;
        }