Utilizo o componente do Excel, abro um Workbook e crio um Worksheet.
Excel.Application excelApp = new Excel.Application();
excelApp.Visible = false;
Excel.Workbook excelWB = excelApp.Workbooks.Open(System.Configuration.ConfigurationManager.AppSettings["CaminhoArquivo"].ToString() + "PlanilhaCusto.xls", System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing, System.Type.Missing);
Excel.Worksheet excelWS = (Excel.Worksheet)excelWB.Worksheets["Plan1"];