public async Task <IActionResult> AddBookAsync(string isbn)
{
Book book = await BookService.AddByISBNAsync(isbn);
if (book == null)
{
return(HttpBadRequest("the provided ISBN does not resolve to a book in our backend service"));
}
var resource = await BookAssembler.ConvertToResourceAsync(book);
return(CreatedAtRoute("api", new { id = book.Id }, resource));
}