Inside Mac OS X
Apple has published a PDF document online entitled "Inside Mac OS X: System Overview":
"Inside Mac OS X: System Overview is intended for anyone who wants to develop software for Mac OS X. But it is also a resource for people who are just curious about Mac OS X as a development and deployment platform.
While aimed at developers, it might be of interest to any hard core Mac user just looking to get a better understanding of how Mac OS X works. [Link to PDF]