public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }
    }

        public IHttpActionResult GetBooks()
        {
//всегда попадаем сюда
        }

        private IHttpActionResult GetBook(Guid Id)
        {
        }

function getBooks() {
    $.ajax({
        type: "GET",
        url: "/api/book",
        success: function (data) {
            }
        }
    });
}

function getBook(id) {
    $.ajax({
        type: "GET",
        url: "/api/book/"+id,
        success: function (data) {
        }
    });
}