No, it's not totally different.
Uh, yes it is. I've been writing Objective-C on and off over the last few months...
[root setVariable:null];
Is radically different from
root.variable = null;
Same with:
root.subtree.subtree.variable = null;
versus
[root [subtree [subtree setVariable:null]]];
If it wasn't different, they'd have no issue just letting people use C++.
But in short, C and Objective-C are radically different. Any C code works in Obj-C. That is all the similarity there is.
If you want a book for developing for the iPhone, check out
Beginning iPhone 3 Development: Exploring the iPhone SDK by Mark/LaMarche. It's really amazing and doesn't bother teaching you the basics (C/Obj-C) and deals strictly with the SDK/iPhone. I didn't know Obj-C but if you have half a brain you can figure out the language as you go.