use the following function to get data as DATATABLE object for N'th sheet :
public DataTable GetWorkSheet(int workSheetID)
    {
        string pathOfExcelFile = fileFullName;
        DataTable dt = new DataTable();
        try
        {
            excel.Application excelApp = new excel.Application();
            excelApp.DisplayAlerts = false; //Don't want Excel to display error messageboxes
            excel.Workbook workbook = excelApp.Workbooks.Open(pathOfExcelFile, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing); //This opens the file
            excel.Worksheet sheet = (excel.Worksheet)workbook.Sheets.get_Item(workSheetID); //Get the first sheet in the file
            int lastRow = sheet.Cells.SpecialCells(excel.XlCellType.xlCellTypeLastCell, Type.Missing).Row;
            int lastColumn = sheet.Cells.SpecialCells(excel.XlCellType.xlCellTypeLastCell, Type.Missing).Column;
            excel.Range oRange = sheet.get_Range(sheet.Cells[1, 1], sheet.Cells[lastRow, lastColumn]);//("A1",lastColumnIndex + lastRow.ToString());
            oRange.EntireColumn.AutoFit();
            for (int i = 0; i < oRange.Columns.Count; i++)
            {
                dt.Columns.Add("a" + i.ToString());
            }
            object[,] cellValues = (object[,])oRange.Value2;
            object[] values = new object[lastColumn];
            for (int i = 1; i <= lastRow; i++)
            {
                for (int j = 0; j < dt.Columns.Count; j++)
                {
                    values[j] = cellValues[i, j + 1];
                }
                dt.Rows.Add(values);
            }
            workbook.Close(false, Type.Missing, Type.Missing);
            excelApp.Quit();
        }
        catch (Exception ex)
        {
            System.Windows.Forms.MessageBox.Show(ex.Message, "Error", System.Windows.Forms.MessageBoxButtons.OK, System.Windows.Forms.MessageBoxIcon.Error);
        }
        return dt;
    }