PEAR Spreadsheet_Excel_Writer

Hi, I create four worksheets which I use for financial data which come from $_POST..within three worksheets I used writeFormula() to calculate something. in the fourth I want to sum up one cell from the previous three worksheets concerning assets calucation for a company. exist a Function in the Spreadsheet_Excel_Writer Class which can only read a value from a already filled cell (with write()) inside a worksheet? Ì only found function which could write data to cells, but not read. I would be very happy if anyone could answer! have a nice day and best regards Sabine