I suggest you read both 1 and 3. They cover a lot of things in Core Data, and they are very complete. They are not upgraded to the iOS 5 API, but I think they provide basic and intermediate concepts to understand what is happening with this infrastructure.
Marcus Zarra (2) is not updating to the new iOS 5 API, and I also think that it can no longer be purchased. But still valid.
In addition, you can also find valuable tutorials on Cocoa is my girlfriend on the Marcus Zarra blog or raywenderlich .
See also ideveloper.tv . The site sells video on Core Data (and much more). Material updated to reflect iOS 5 API and explained by Marcus Zarra .
Finally, you can also see https://stackoverflow.com/questions/1939/how-to-articles-for-iphone-development-and-objective-c .
Hope this helps.
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