Although this book is well written in concept, the editing of the final product is awful. There are a great deal of grammatical errors which seemed to me a basic word-processor would have caught.
The book should be titled, more correctly, "Unix Programming for OS X." Unix system programmers will find almost nothing new in this book (gcc, gdb, file-systems, signals, libraries, etc. - although, the Objective-C examples can be helpful in understanding how to implement things in new ways, and also the chapter on Subversion was a nice introduction for me.)
If you are just learning how to program on a Unix platform, I'd recommend this book unequivocally. I paid full price for mine ($70), without taking a hard enough look at the contents and found myself with an expensive, redundant book on my shelf.