I do't know exactly what you mean about OSX internals. But if you want to develop GUI based applications than the OSX developer docs are where you want to start, especially Cocoa which is the GUI framework. (Objective-C is the main OSX programming language.)
That's still the gold standard when it comes to OS X internals. I don't know of anything that is more current.
Another good but dated book is Mac OS X for Unix Geeks (Leopard) If you're comming from a Unix background it will help tie up some of the loose ends. I know it helped me.
Currently Being ModeratedApr 29, 2012 10:53 AM (in response to Keith Barkley)
I see now I wasn't very specific. Apologies for that. I'm not looking at UI development just yet (I don't get much time to code on my personal time). Right now my main concern is ramping up on what has become basics to me in linux/windows land in terms of admistering my own and family's systems. I found a lot of user level help/documentation on various forums but not a whole lot in terms of implementation details. I know I can eventually dig all this up but I thought there might have been a primer or two out there. Thanks again!
Mac OS X and iOS Internals: To the Apple's Core might be a contender to replace Singh's book as the standard reference...if they ever get around to releasing it. And of course we won't know until it finally makes it to the shelves.
It think the date has been pushed back a few times in order to try and stay up to date when it's finally released - unfortunately that's a losing proposition. So my guess would be it either really comes out this summer...or Mountain Lion (and maybe an iOS update) push it back into vapourware again...