The Stanford University class on iTunesU would be a good place to start.
I agree with this. I quite like the CS193 course on iTunes.
My start was I was working in film special effects software and a couple of colleagues introduced me to NeXTSTEP and Objective-C (as well as AmigaOS and Intuition) in the early 1990s, I didn't appreciate it for what is was at the time, I was a C++ Silicon Graphics guy at the time. When OS-X came out around the turn of the millenium, I tried to get back into it but it still didn't quite stick.
I got a much much earlier version of this book... Amazon -
Programming in Objective-C (5th Edition) (Developer's Library): Stephen G. Kochan: 9780321887283: Amazon.com: Books )
Publication Date: December 14, 2012 | ISBN-10: 032188728X | ISBN-13: 978-0321887283 | Edition: 5
Programming in Objective-C, Fifth Edition
Updated for OS X Mountain Lion, iOS 6, and Xcode 4.5
After a while I fell in love with Objective-C and the related object model (coming from more than a decade of C++), I got so bored doing C++ poker machine code on embedded Linux I grabbed everything I could find on Objective-C, several books, every web site on anything google came up with... I started playing around with Objective-C code on Linux with gcc. Created huge class libraries and elaborate data structures etc... Googling around constantly on Objective-C features and following every Objective-C tag on stack overflow...
When I worked somewhere that also had an iPhone code base, I delved into the code and pulled it a part and tried to figure out why the developers did things in a certain way and finally after getting canned one time... I took a few months off and got up every morning went to code like it was a job, churning out app after app, I didn't even seriously look for another job until the following year.
Now, I have an Objective-C job and a fairly large corporate code base to dig around in.. ;-)
R.