I learn plain old C by reading Kochan's "Programming in C". He also have a "Programming in Objective-C 2.0" (ObjC 2.0 seems to be where GNUstep is going, so I don't see why you can start with that), which is probably what you want to go with. Etoile's David Chisnall (hope I didn't misspell that) recently wrote a Cocoa book as well, I'll let him go in more detail.