Concurrent-Programming-in-Mac-OS-X-and-iOS is a project mainly written in Objective-C, it's free.
O'Reilly's Concurrent Programming in Mac OS X and iOS Example Codes
This repository contains all example codes from the O'Reilly book "Concurrent Programming in Mac OS X and iOS" by Vandad Nahavandipoor
You can find the book on O'Reilly: http://oreilly.com/catalog/0636920020332
Or on Amazon: http://www.amazon.com/Concurrent-Programming-Mac-iOS-Performance/dp/1449305636/ref=sr_1_1?ie=UTF8&qid=1308384608&sr=8-1
Or other major book sellers