Programming in Objective-C - Kochan (http://www.amazon.com/Programming-Objective-C-4th-Developers...)
I am currently halfway through this book, it is primarily focused on the actual Objective-C language, and almost nothing on actual iOS/Cocoa Development (some small bits in the later chapters). I was recommended this to learn the underlying language before jumping in, very enjoyable so far, the exercises are great.
I have heard good things about both Nerd Ranch books (http://www.amazon.com/iOS-Programming-Ranch-Guide-Guides/dp/...) and will likely use that for introduction to Cocoa and iOS SDK.
Originally I planned on using "Beginning iOS 5 Development" from Apress as the iOS SDK learning book (http://www.amazon.com/Beginning-iOS-Development-Exploring-SD...), but it seems that the reviews are quite low compared to previous versions.
Finally, if you enjoy learning through video, the Stanford course is very highly recommended, through iTunes (http://itunes.apple.com/us/itunes-u/ipad-iphone-application-...). I found them very useful to start with, but I felt I wasn't learning much syntax through the videos, the talking portions are very in depth, but the coding portions are very quick (and went over my head due to not knowing Objective-C!). I will likely return to these when I have a working understanding of Objective-C.
(Most of these tips from my understanding of: http://stackoverflow.com/questions/3403049/best-book-resourc... and http://stackoverflow.com/questions/1939/how-to-articles-for-...)
If you want an intensive study of Objective C itself, look at the Kochan book. It's the bible.
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