If you know C/C++ and want to get into Objective-C I recommend the "Objective-C Guide for C++ Programmers": http://ktd.club.fr/programmation/fichiers/cpp-objc-en.pdf

It goes through step by step and explains how the OO terms you know map from C++ to Objective-C, e.g., "If you want to declare a static class method, you use this syntax..." If you're familiar with OO in general it's pretty good, too.

It really lowers the Objective-C syntax hurdle, IMO.