Recently just started to read a few pages in this book, Mac OS X Internals: A Systems Approach, by Amit Singh. It’s a bit old, too much contents about the old PPC.
Then yesterday found an interesting video about same topic, YouTube: Inside the Mac OS X Kernel [24c3]. (Note: some people can’t access it.) This video was public recently, but it’s also a bit old, Dec. 2007. It’s from 24th Chaos Communication Congress: Inside the Mac OS X Kernel, by lucy. (You can find the slides there.)
And I am not sure whether one sentence in the presentation is still correct or not now, “The Mac OS X kernel is not 64 bit.”