here are some resources I've compiled during my time trying to learn how OS X operates. These are mostly oriented toward systems-level OS X stuff and programming for OS X (pronounced "OhEs Ten", not "OhEs Ecks").
- Mac OS X Internals, A Systems Approach by Amit Singh
pretty much THE best and only OS X internals book out there. Buy this if you are doing any real digging into OS X.
CocoaDevCentral.com and Cocoadev.com - great sites for starting out and learning Objective C and Cocoa. Once you get some Obj-C under your belt, check #3 out - it's a reference for the Objective-C language.
The Objective-C Programming Language
great, free reference, but it's better to read this after reading the above sites and fooling around. It's kind of dry.
- kernelthread.com - by Amit Singh. equivalent to Sysinternals writings for Windows or Linux Kernel Mailing List for Linux.