How to write serialized objects to XML files without overwriting? SAXParseException

So, as the title suggests, I've been trying to write serialized objects without overwriting the file. The problem is that whenever I add an object to this file I get the following sentence:

"org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file. Continuing ... "

And then nothing is written to the file. If I try to serialize the objects into a file other than XML, let's assume the type .ser, returns me an error saying that it could not be written to the file.

 public static void addRecords() throws IOException {

    inpt = new XMLDecoder(
            new BufferedInputStream(
                    new FileInputStream("library.xml")));
    ArrayList<Book> books;

    Scanner input = new Scanner(System.in);

    System.out.printf("%s%n%s%n? ",
            "Enter ISBN number, Author's name, Book's Title.",
            "Enter end-of-file indicator to end input.");

    while (input.hasNext()) {

        try {
            Book record = new Book(input.nextLine(),
                    input.nextLine(), input.nextLine());

            books = (ArrayList<Book>) inpt.readObject(); //Aqui o Intellij "seleciona" essa linha e me diz o seguinte:"Unchecked cast: 'java.lang.Object' to 'java.util.ArrayList'"
            inpt.close();
            books.add(record);
            output.writeObject(books);

        } catch (NullPointerException e){
            e.getStackTrace();

        } catch (ArrayIndexOutOfBoundsException arrayExc){

        }
        System.out.println("? ");
    }
}