
Hillegass is basically bible for Cocoa (Mac) programming. But, its not the best way to learn Cocoa Touch (iOS) for iOS. For that, I'll second Skroob's recommendation of Craig Hockenberry's book. Also, Jeff LaMarche and David Mark's Beginning iPhone Development is rather good and I personally have a nice copy of Bill Dudney and Chris Adamson's iPhone SDK Development that I'll skim on occasion.
But, you don't need to read all three of them; they basically cover the same material in different (but equally good) ways. After you've worked your way through one of them, maybe pick up Erica Sadun's The iPhone Developer Cookbook or iPhone Cool Projects by Wolfgang Ante, Mike Ash and a bunch of other really smart people.
And at some point, move away from the path documented and write your own app!
Jeff LaMarche and David Mark's Beginning iPhone Development, Bill Dudney and Chris Adamson's iPhone SDK Development or Erica Sadun's iPhone Developers Cookbook are all very good books to look into (although, I vaguely remember hearing something like that Erica is working on another edition that covers iPad additions. May be worth holding off on this particular book for a bit..).
The problem is, the really fun stuff (Game Center, multitasking, and iAd to name some; there's many more fun things that are.) is still under NDA. Or maybe there's not many printed books written on the topic, period (Blocks/GCD, AddressBook, using Instruments to debug/optimize your app, etc). So you want to figure out how to read the docs and guides from Apple as quickly as you can. Which, luckily, isn't too bad to do — the docs tend to be very well written.
http://www.amazon.com/iPhone-SDK-Development-Pragmatic-Progr...
Will def check out Cocoa Design Patterns when I get a chance.