Newtonsoft.Json.dll or enter Newtonsoft or json.net online via ->Tools->Library Package Management Tools->NuGet Management Packages->
Newtonsoft.Json is ok:
context.Response.ContentType = "text/plain";
BooksService service = new BooksService();
List<Books> list=service.GetAll().Take(5).ToList();
JsonSerializerSettings setting = new JsonSerializerSettings()
{
ReferenceLoopHandling = ReferenceLoopHandling.Ignore
};
strintret = JsonConvert.SerializeObject(list, setting);
context.Response.Write(ret);