public void librarian_menu() throws Exception{
		int lib_menu;
		System.out.println("------------------------------");
		System.out.println("1.  ");
		System.out.println("2.   Ʈ");
		System.out.println("3.   ");
		System.out.println("4. α׾ƿ");
		System.out.println("------------------------------");
		System.out.print("޴ Է:");
		lib_menu = scan.nextInt();

		switch(lib_menu){
		case 1:
			book_register();//
			break;
		case 2:
			book_update(); // Ʈ
			break;
		case 3:
			book_delete(); // 
			break;
		case 4:
			logout(); //α׾ƿ
			break;
		}
	}