Objective-C is easy to learn, especially if you already know C. Objective-C is a superset of C that adds a thin layer of object-oriented features. Those features allow us to define classes with instance and class methods that look a lot like methods in Ruby, but these methods are fast because they are written in C. And unlike Java and C++, we don't need a shelf-full of books to use as a reference.
Here are the three most important books for an Objective-C programmer to own.
If you are comfortable with C, then just invest ten dollars in Andrew Duncan's Objective-C Pocket Reference (O'Reilly). You can read it in an afternoon, and it contains all the Objective-C you'll need to know to use Nu.
If you want some help with C as well, then I recommend Stephen Kochan's Programming in Objective-C. Experienced C programmers may find it a bit verbose, but it is comprehensive and accurate.
Graham Lee's comp.lang.objective-C FAQ.