Ok  i understand your problem now :)  .... loads of stuff to do make jqgrid  to work with webmethod 

firstly download JSON plugin from this link 

http://code.google.com/p/jquery-json/downloads/detail?name=jquery.json-2.2.min.js

then your  jquery code should look like this

// change these references as per your own solution
 <script src="Scripts/jquery-1.4.1.js" type="text/javascript"></script>
    <script src="Scripts/jquery.json-2.2.min.js" type="text/javascript"></script>
    <script src="Scripts/jquery-ui-1.8.5.custom.min.js" type="text/javascript"></script>
    <script src="src/i18n/grid.locale-en.js" type="text/javascript"></script>
    <script src="src/jquery.jqGrid.min.js" type="text/javascript"></script>


  <script>


$(document.ready(){
// Update default settings
//required to make it work for JSON data handling
// this just overrides ajax settings and datatype
$.extend($.jgrid.defaults,
         {datatype: 'json'},
         {ajaxGridOptions: { contentType: "application/json" ,
           success: function(data, textStatus) {
            if (textStatus == "success") {
                var thegrid = $("#Grid1")[0];
                thegrid.addJSONData(data.d);
                thegrid.grid.hDiv.loading = false;
                switch (thegrid.p.loadui) {
                    case "disable":
                        break;
                    case "enable":
                        $("#load_" + thegrid.p.id).hide();
                        break;
                    case "block":
                        $("#lui_" + thegrid.p.id).hide();
                        $("#load_" + thegrid.p.id).hide();
                        break;
                }
            }
        }
         }
                 

    });
          jQuery("#Grid1").jqGrid({
              url: 'WebForm3.aspx/GetDataTable',
               dataType: "json",
              mtype: 'POST',
              colNames: ['BookID', 'BookName'],
              colModel: [{ name: 'BookID', index: 'BookID', width: 200 },
                   { name: 'BookName', index: 'BookName', width: 300}],
              pager: '#pager',
              sortname: 'id',
              viewrecoreds: true,
              imgpath: 'Themes/images',
              serializeGridData: function(data)
               {
                return  $.toJSON(data);   /// done to override default serialization
                }
                        

          }).navGrid("#pager", { edit: true, add: true, del: true, refresh: true, search: true });


      });
  </script>

Now  update the webmethod  definition to this

[WebMethod]        
        public static s_GridResult GetDataTable(string _search ,string nd ,int rows ,int page ,string  sidx , string sord )
        {  

 int startindex = (page - 1) * pagesize;
            int endindex = page * pagesize;
            string sql = @" WITH PAGED_CUSTOMERS AS
                      {
                          SELECT BookID,BookName
                          ROW_NUMBER() OVER(ORDER BY " + sidx + @" " + sord + @") AS ROWNUMBER
                          FROM BOOKS}";
            DataTable dt = new DataTable();
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Data Source=.;Initial Catalog=library;Integrated Security=True"].ConnectionString);
            SqlDataAdapter adapter = new SqlDataAdapter(sql, conn);
            var rows = adapter.Fill(dt);
            s_GridResult result = new s_GridResult();
            List<s_RowData> rowsadded = new List<s_RowData>();
            int idx = 1;
            foreach (DataRow row in dt.Rows)
            {                  
                  s_RowData newrow = new s_RowData();
                  newrow.id = idx++;
                  newrow.cell = new string[2];  //total number of columns  
                  newrow.cell[0] = row[0].ToString();
                  newrow.cell[1] = row[1].ToString();
                  rowsadded.Add(newrow);                    
            }
            result.rows = rowsadded.ToArray();
            result.page = page;
            result.total = dt.Rows.Count;
            result.record = rowsadded.Count;
            return result;
}

Hope this works for u now :) let me know if you get stuck up on something