public IActionResult OnPost()
        {
            if (!ModelState.IsValid)
            {
                this.Categories = HtmlHelper.GetEnumSelectList <Category> ();
                return(Page());
            }

            if (Book.ISN > 0)
            {
                BookData.Update(Book);
            }
            else
            {
                BookData.Add(Book);
            }
            BookData.Commit();
            TempData["Message"] = "Book Saved";
            return(RedirectToPage("./Detail", new { isn = Book.ISN }));
        }