Some people have been curious as to what are the resources that I’ve used to pick up Objective-C in a relatively short amount of time (about 3 months, including the time taken to develop and ship the first release of Due in September 2010). There are two books have been indispensable in my learning that I’d recommend to anyone picking up iOS development:
Programming in Objective-C 2.0 (3rd Edition) by Stephen G. Kochan
Best introduction to Objective-C. Mostly command-line (so it can be quite boring), but that takes away the distraction from learning the language as the learning curve is really steep (mostly in memory management). If you’ve some programming background, you don’t need to go through the entire book, and you’ll know which to skip.
iOS Programming: The Big Nerd Ranch Guide (2nd Edition)
This runs you through the graphical user interface development (the fun part) on the iPhone. But I really recommend to start with Kochan’s book first as the language learning curve is steep. Having the GUI stuff coming in can be even more confusing for someone new to programming or Objective-C.
Updated both title and and links with newer 2011 editions.