public void IsbnEntered()
{
    var isbn = view.Isbn;

    if (isbnService.NumberIsValid(isbn))
    {
        var details = isbnService.RetrieveDetailsForIsbn(isbn);

        if (details != null)
        {
            view.Display(details);
            view.EnableSaveButton();
        }
        else
        {
            view.DisplayError("ISBN could not be found");
        }
    }
    else
    {
        view.DisplayError("Invalid ISBN");
    }
}