Microsoft has finally publically released the specifications for the Word, Excel and Powerpoint and Office Drawing formats (97-2007)
Previously you had to write to Microsoft to obtain them.
http://www.microsoft.com/interop/docs/officebinaryformats.mspx
Microsoft Word
Word 97-2007 Binary File Format (.doc) Specification PDF | XPS
Microsoft PowerPoint
PowerPoint 97-2007 Binary File Format (.ppt) Specification PDF | XPS
Microsoft Excel
Excel 97-2007 Binary File Format (.xls) Specification PDF | XPS
Excel 2007 Binary File Format (.xlsb) Specification PDF | XPS
Office Drawing
Office Drawing 97-2007 Binary Format Specification PDF | XPS