FreeXL  1.0.6
File format specifications and source information

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.