(This review is about the 1986 edition, not the 1991. I assume big changes, but either way I haven't read it.)
Okay, so you just got a Mac or an iPhone and you've noticed that the development tools you need are free (and, indeed, come with the computer). You go out and buy books, but you want to see the original book. Well, this is it. Congratulations, you've just purchased an interesting historical document that has almost nothing to do with Mac programming, in a language that is almost but not entirely unlike the Objective-C you'll be using to write your app.
The 1986 edition is truly a time warp -- for example, the choice of "acetate" as an analogy to describe a GUI view is probably going to be lost on anyone who's only ever done page layout on a computer. Cox writes comparisons of Objective-C to Ada, C++, and Smalltalk, but the comparisons are far, far outdated. Three years before the original ANSI C standard came out, Cox was still using K&R C as his substrate language. The coverage of how object-oriented GUI systems work is more or less on target, but since it's based on a very old version of X, it isn't very much like the OpenStep/Cocoa environment. But a bigger problem than its antiquatedness is the fact that (probably by necessity) it's three parts textbook, one part advertisement for Stepstone's (or at the time, PPI's) product. The grating and poorly-thought-through term "Software-IC" (for a binary object library) pops up everywhere.
Used copies, however, can be had pretty cheaply, so if you like computer archaeology it's certainly a nice little trip to the days when object-oriented programming was just going mainstream and Steve Jobs was looking for technologies to build his NeXT system on. There's also enough source code to learn a bit about writing your own Objective-C libraries, if you know how to translate to the @ syntax that ObjC has used since not long after this book came out. For the most part, though, unless you're a serious Mac or programming languages historian, it's not worth going out of your way for.