From what I've seen of the book, much of it is dedicated to UNIX "systems" programming, and the book could almost be renamed 'UNIX programming for OS X". That is, it covers UNIX signals, terminal I/O, IPC, BSD sockets, pthreads, the GNU compiler suite and Apple's extensions to it, etc. Much of this and more is covered in greater depth in the standard reference for the subject: Steven's 'Advanced Programming in the UNIX environment', but the exposition in 'Advanced Mac OS X Programming' is more streamlined and cherry picked.
The audience for the book seems to be mainly those who may have some experience developing in some high level language or framework like Cocoa or Java and who want to delve deeper into OS X's Darwin layer. That being so, given the 2005 publishing date, it wouldn't be in any way out of date, even those parts dealing with Apple's proprietary APIs (Core Foundation, etc.), which are the only things that would have changed in the interim (and they haven't). All told, it's a valuable complement to those OS X books dealing strictly with Cocoa/Obj-C.