public async void PostBookAsync()
 {
     var message = new ValueSet();
     message.Add("command", "POST");
     string json = new Book { Title = NewBookTitle, Publisher = NewBookPublisher }.ToJson();
     message.Add("book", json);
     string result = await SendMessageAsync(message);
 }
 public void AddBook(Book book)
 {
     _books.Add(book);
 }