The .xls binary file format is extensively documented and is publicly available.
The most authoritative source is made available by Microsoft at http://msdn.microsoft.com/en-us/library/cc313154%28v=office.12%29.aspx
A simpler option is made available by Open Office: http://sc.openoffice.org/excelfileformat.pdf
Searching the web you'll easily find several other valuable information sources.