In Panther there is a AddressBook Pallete for InterfaceBuilder that displays the AddressBook Content like in Mail, and iCal. It provides a view known as the people picker.
Classes that relate to the AddressBook:
MacDevCenter has an article about the AddressBook in the "Learning Cocoa" column: (http://www.macdevcenter.com/pub/a/mac/2002/08/27/cocoa.html?page=1)
Sharing addresses with OS X 10.2 server via LDAP: http://www.afp548.com/Articles/Jaguar/emailsharing.html
AddressBook is built off of the MetaKit.