last object serialized gets overwritten

I am writing a program for keeping track of library books. There is an object book that includes a title, sku number, price and quantity. All the books are stored in an Array List. I'm trying to serialize the books and add new books but every time a book is added the last is overwritten.

here is the code below to load objects from save

public static void readSave() {
		
		File stockFile = new File("inventory.txt");
		
		try {
			if(!stockFile.createNewFile() && stockFile.length() != 0) {
				ObjectInputStream objIn = new ObjectInputStream(new FileInputStream(stockFile));
				int size = objIn.readInt();
				
				for(int i = 0; i < size; i++) {
					Book t = (Book) objIn.readObject();
					bookList.add(t);
				}
				
				objIn.close();
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}	
	}

here is my save method and it is set to save every time the program executes.

public static void save() {
		File inventoryFile = new File("inventory.txt");
		
		ObjectOutputStream objOut;
		try {
			objOut = new ObjectOutputStream(new FileOutputStream(inventoryFile));
		
			objOut.writeInt(bookList.size());
			Iterator<Book> i = bookList.iterator();
			while(i.hasNext()){
				objOut.writeObject(i.next());
			}
		
			objOut.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

below is my main method which calls these functions

public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					readSave();
					CampusBookWindow window = new CampusBookWindow();
					window.frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
				save();
			}
		});
	}

My problem was when I hit the add book button I would not create a new book object for that item, but instead I was reassigning that last book object so the object last book object was erased from the bookList array list