public ICourse AddBook( string isbn )
        {
            const string ROUTE = "";

            var bookIsbn = new Valence.IsbnAssociation { Isbn = isbn };

            var client = CreateClient();

            var request = CreateContentRequest( ROUTE, Method.POST );

            request.AddBody( bookIsbn );

            var response = client.Execute<Valence.Book>( request );

            if(( response.StatusCode != HttpStatusCode.OK ) || ( response.Data.Isbn != isbn )) {
                throw new InvalidOperationException( "Unable to add isbn to course. Returned status: " + response.StatusCode );
            }

            return this;
        }
        public ICourse AddBook(string isbn)
        {
            const string ROUTE = "";

            var bookIsbn = new Valence.IsbnAssociation {
                Isbn = isbn
            };

            var client = CreateClient();

            var request = CreateContentRequest(ROUTE, Method.POST);

            request.AddBody(bookIsbn);

            var response = client.Execute <Valence.Book>(request);

            if ((response.StatusCode != HttpStatusCode.OK) || (response.Data.Isbn != isbn))
            {
                throw new InvalidOperationException("Unable to add isbn to course. Returned status: " + response.StatusCode);
            }

            return(this);
        }