// final JTree tree = new JTree(createNodes());

tree.addTreeSelectionListener(new TreeSelectionListener() {
    @Override
    public void valueChanged(TreeSelectionEvent e) {
        DefaultMutableTreeNode node = (DefaultMutableTreeNode) tree.getLastSelectedPathComponent();
        if (node == null) {
            // Nothing is selected.
            return;
        }
        if (node.isLeaf()) {
            Book book = (Book) node.getUserObject();
            System.out.println(book.getTitle());
        }
    }
});