An Objective-C tutorial is outside the scope of this manual (currently), but there are some useful resources online.
Learn Objective-C, Tutorialspoint
Objective-C Quick Guide looks like a good tutorial start. Ignore the parts about “gcc”, “MacOSX”, “GNUStep” though. They also have a much larger tutorial, where you should ignore “Blocks”, “Posing”, and anything mentioning “ARC”.
Objective-C Tutorial, Codecrackers
Objective-C Tutorial is basically the same as whats offered at Tutorialspoint, but there are differences. Someone plagiarized but I can’t tell who did it. The codecrackers page looks a bit nicer.
Objective-C Tutorial, Etutorials
As part of introducing Cocoa there is also an Objective-C tutorial.
Programming with Objective-C, by Apple
Programming With Objective C covers all the basics. Ignore the chapter about “Blocks”, everything else should be fine.
Free Objective-C Books
onlineprogrammingbooks has a list of downloadable book PDFs.
Concepts in Objective-C Programming, by Apple
Concepts in Objective-C Programming is more in-depth and is a good read to understand some terminology used by fellow Objective-C coders. Ignore the chapters “Delegates and Data Sources”, “Model View Controller”, “Object Modeling”, “Outlets”, “Receptionist Pattern”, “Target-Action”, “Toll-Free Bridging”
Learn Objective-C, CocoaDev
Learn Objective-C gives a quick overview of concepts. Ignore the part about “Dot Syntax” though.
Learn C for Cocoa, CocoaDev
Learn C for Cocoa is a bit outdated but an easy read.
An interactive tutorial site to learn the basics of C is learn-c.org free interactive C tutorial
Now that you know Objective-C, your are well prepared for the MulleObjC basics.