Here is a list of all related documentation pages:

The background story for FreeXL	Where, when and why a new free software library was born..
File format specifications and source information	The .xls binary file format is extensively documented and is publicly available
About the .xls binary format	What a .xls binary file really is
Other tools and libraries	There an impressively wide choice of Free and open source libraries and tools supporting the .xls format