It's old (circa 2006) and really low level, but I loved reading through Mac OS X Internals (http://www.osxbook.com http://www.osxbook.com) If it were updated,

It's old (circa 2006) and really low level, but I loved reading through Mac OS X Internals (http://www.osxbook.com http://www.osxbook.com) If it were updated, or a vol2 was released with all the new stuff, I would immediately buy it. It's not going to teach you how to use Finder, but you'll have a great understanding of the internals.