Objective-C 2.0 introduced garbage collection and "properties", as well as a new enumeration syntax.

The best beginning Cocoa book is "Cocoa Programming for Mac OS X", found here: http://www.amazon.com/exec/obidos/ASIN/0321503619/bignerdran...

It's a great book, and a huge number of mac developers got started with it. The new version (which has Obj-C 2.0, Leopard, and other updates) just came out a few weeks ago.