public book GetBookById(string title_id)
        {
            IRepository <book> book_repo = Repos.get_book_repo();

            book book = book_repo.Find(title_id);

            return(book);
        }
        public IEnumerable <book> GetBooks()
        {
            IRepository <book> book_repo = Repos.get_book_repo();

            List <book> books = book_repo.FindAll();

            return(books);
        }