title: What are good sources of knowledge / information about the implementation of objective-C and cocoa/ cocoa-touch?
body: <p>I've been developing for iOS for some time now, I think I have a good grasp of the cocoa-touch framework but I feel I am missing some nuggets about the underlying implementation of objective-C and cocoa-touch. e.g. recently when having an issue with allocations several posters pointed out some of the finer points of iOS implementation that made the issue I was seeing much clearer (and meant I understood how to resolve the issue properly) </p>
  
  <p>I would like to know of any books, online-references or just good sources of knowledge providing an insight into the underlying implementation of objective-C and cocoa-touch that would allow me to gain a greater and more complete understanding of the framework, platform and language, under the hood. Anyone ?? </p>