If you've never programmed in C, you may want to seek out a good C tutorial. You can get pretty far in iOS working only with Objective-C objects, but plenty of C-isms leak through so it helps to be familiar with pointers, structs, functions and memory management.
Objective-C isn't too hard to get up to speed with, especially if you've ever worked in both C and a dynamic language like Python or Ruby. Wrapping your head around Cocoa Touch is a little more daunting, but Sadun's book does a good job of building up the basics step by step, then providing chapters on specific frameworks you can page-fault in as needed.
Apple recently added "Start Developing iOS Apps Today" https://developer.apple.com/library/ios/#referencelibrary/Ge... which is probably worth taking a look for a beginner.
Fresh book recommendations delivered straight to your inbox every Thursday.