For general multi threading books I only have gotten <a href="http://www.amazon.com/gp/product/0201633922?ie=UTF8&tag=coliwhee-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=0201633922">Programming with POSIX(R) Threads</a>

However this is for the old POSIX API's. In general I would read up on Grand Central Dispatch and NSOperation
http://developer.apple.com/technologies/mac/snowleopard/gcd.htmlhttp://developer.apple.com/library/mac/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html
http://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/GCD_libdispatch_Ref.pdf
http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/NSOperation_class/Reference/Reference.html
http://developer.apple.com/cocoa/managingconcurrency.html
http://developer.apple.com/library/mac/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html

Between blocks with grand central dispatch and NSOperation, you've got some incredibly powerful and easy multithreading tools. However beware, just because it's easy it's still dangerous at times. There is a reason multithreading is compared to juggling chainsaws.