I use Objective-C for iPhone app development. Now, I want to tackle C so that I can build modules for Python. How do I go about doing this?
Get some books, K&R old but classic, C in a nutshell (O'Reilly) is good and cover C99.
I'd avoid reading K&R: its coverage of C is frankly naive. You want something that isn't afraid to dissect C's many warts and shortcomings and discuss how best to work around them in real-world use.
Honestly, if the OP already knows ObjC then the main task is learn their way around Python's C API. There's no need to muck around with malloc/free and other clunky C-isms unless you're wrapping C libraries that require their use.