threading in Objective-C

Here is an overview from Apple.

This is a book you have to pay for, but Advanced Mac OS X Programming is a really good reference for this sort of thing (kind of low-level C/UNIX-y stuff, but specifically related to OS X and in many parts, Cocoa).

Generally you probably want to avoid multithreading your app if possible. There are some tools you can use, such as NSTimer, NSTask, and NSOperation/NSOperationQueue, to do many multithread-like things without actually threading (or at least simplifying/hiding it). Your debugging can get a lot more complicated once you start multithreading, as you probably know if you've done any before. But anyway, here is a really nice article on Cocoa threading in Leopard from Cocoa Samurai.