Wiki
TrueType
http://en.wikipedia.org/wiki/TrueType
OpenType
http://en.wikipedia.org/wiki/OpenType
Microsoft Typography
http://www.microsoft.com/typography/default.mspx
Font specifications
http://www.microsoft.com/typography/SpecificationsOverview.mspx
, including OpenType and TrueType.
Font Tools
http://www.microsoft.com/typography/tools/tools.aspx
Apple
TrueType reference manual
http://developer.apple.com/fonts/TTRefMan
Adobe
OpenType
http://www.adobe.com/type/opentype/