Getting null value instead of data in web API 2

public static class WebApiConfig
   {
       public static void Register(HttpConfiguration config)
       {
           config.MapHttpAttributeRoutes();

           config.Routes.MapHttpRoute(
               name: "DefaultApi",
               routeTemplate: "api/{controller}/{isbn}",
               defaults: new { isbn = RouteParameter.Optional }
           );
       }

public class ServiceAController : ApiController
   {


       [HttpGet]
       public Book GetBook(string isbn)
         {

           using (AppDbContext db = new AppDbContext())

           {

               var query = from b in db.Books

                           where b.ISBN == isbn && b.Source == "Book Store 1"

                           select b;

               return query.SingleOrDefault();

           }

       }
   }