some years ago I worked on a spreadsheet project. Back then the excel file format was documented. This allowed us to read/write natively to the files. I have no idea if it still is, but if it is, you will certainly find the documentation at microsoft.com eh?
Otherwise the COM route might be what your looking for. To work with Excel using the COM interface you will find documentation at msdn.microsoft.com for the "Excel Object Model".
good luck
-pete