@EventHandler
    public void onInventoryChangeEvent(InventoryClickEvent ev) {
        if (ev.getInventory().getName().equalsIgnoreCase("BookShelf")) {
            if (ev.getCurrentItem() != null && ev.getCursor() != null) {
                if (!ev.getCurrentItem().getType().equals(Material.WRITTEN_BOOK) && !ev.getCursor().getType().equals(Material.WRITTEN_BOOK)) {
                    ev.setCancelled(true);
                }
            }

        }
    }