probleming writing multiple items to text file

Hello all,
I am taking a graduate seminar that involves Java. I am working on an assignment where I create an interface that will capture a books title, author, and availability. The end result is to write this to a file 5 times. However, I am finding that each time I press the "save" button on my interface, it writes to the text file but replaces the text that was there before.

Code Java:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.ObjectOutputStream;
 
public class TryThis extends JFrame {
 
	private JPanel panel;
	private JButton button;
	private JCheckBox check;
	private JTextField bookTitle;
	private JTextField authorField;
	private JLabel bookLabel;
	private JLabel authorLabel;
	String title;
	String author;
	String available;
 
	public TryThis(String str) {
		super(str);
 
	}
 
	public static void main(String[] args) {
 
		TryThis myGUI = new TryThis("Save book records");
		myGUI.createAndShowGUI();
		// myGUI.writeTofile();
	}
 
	private void createAndShowGUI() {
 
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		Container contentPane = getContentPane();
		contentPane.setLayout(new BorderLayout());
		contentPane.setLayout(null);
 
		JButton button = new JButton("Save");
		button.setBounds(230, 125, 75, 20);
		this.add(button);
		button.addActionListener(new BookEntry());
 
		check = new JCheckBox("Available", false);
		check.setBounds(230, 100, 100, 20);
		this.add(check);
 
		bookLabel = new JLabel("Book title:");
		bookLabel.setBounds(10, 10, 100, 20);
		this.add(bookLabel);
		bookTitle = new JTextField();
		bookTitle.setBounds(120, 10, 350, 20);
		this.add(bookTitle);
 
		authorLabel = new JLabel("Author:");
		authorLabel.setBounds(10, 50, 100, 20);
		this.add(authorLabel);
		authorField = new JTextField();
		authorField.setBounds(120, 50, 350, 20);
		this.add(authorField);
 
		setSize(500, 200);
 
		setVisible(true);
 
	}
 
	public class BookEntry implements ActionListener {
		public void actionPerformed(ActionEvent ev) {
			title = bookTitle.getText();
			author = authorField.getText();
			System.out.println(title);
			System.out.println(author);
			if (check.isSelected()) {
				available = "available";
 
			} else {
				available = "not available";
 
			}
			System.out.println(available);
 
			StringBuilder result = new StringBuilder();
			result.append(title);
			result.append(author);
			result.append(available);
 
			try { // all the I/O stuff must be a in try/catch
				File file = new File("book.txt");
				if (!file.exists()) {
					file.createNewFile();
					System.out.println("new file:" + file);
				}
 
				BufferedWriter writer = new BufferedWriter(new FileWriter(file));
				writer.write(result.toString());
				writer.close();
 
			} catch (Exception ex) {
				ex.printStackTrace();
			}
		}
	}
 
}

What am I doing wrong?

Thank you.