The Apple Developer Connection (ADC) Documentation for Mac OS 8, 9 and OS X with useful links to Darwin, Cocoa, Carbon, Tools and Technology used in the operating system.
http://developer.apple.com/techpubs/macos8/mac8.html
and
http://developer.apple.com/techpubs/macosx/macosx.html
Update: The ADC site has changed to Apple Developer. Below is the link to the Documentation
https://developer.apple.com/resources