I have a copy of the NeXT/Apple Intro to OOP and Objective-C book. It's really quite good. In addition to covering the language itself, it does a nice job of covering the ideas behind OOP.|
The only small problem with the book is that categories and protocols were NeXT extensions that weren't supported in the Stepostone compiler, and aren't supported in the POC. Of course, for most people, that isn't a problem, since they're probably using the NeXT Objective-C version that's in gcc.
After programming in Objective-C, and having to switch to using Java, it amazes me that more people don't use it. Java is sooo clumsy and ugly compared to ObjC. At least once a day I find myself wishing for categories or poseAsClass:.
Objective-C takes more from Smalltalk than message-passing, BTW. The original Stepstone libraries and the old NeXT frameworks were heavily influenced by the Smalltalk class libraries. To some extent, NeXT's still is.