The Office User Interface Blog is quite an interesting read. Among the “must reads”:

• History of Office UI
  From Word 1.0 to Word 2000
• Combating the perception of bloat
  A look at how Office 2000 tried to combat bloat
• Office 12 Keyboard Shortcuts
  KeyTips.
• The MiniBar
  A way of reducing command loops and quick way of changing formatting.
• Dialog Launchers
  Exposing advanced options through ribbon panels.
• Be Willing To Be Wrong

It’s also well worth a look through the archive.