I know that there are a lot of online resources out there for modding like macosxhints.com but sometimes you want something more indepth and you might want to "why" Apple did things a certain way.
I picked up a copy today at my local Chapters bookstore.
I would recommend this to anyone who wants to get serious about modding on the mac. I'd say that it might even be a good intro for development on OS X as you get to study bundled applications and extend their functionality.
The examples are explained in such a way that even a novice should get the hang of it pretty quickly.
Amazon.com link
This is perfect for beginners to modding the GUI or those of us who want to get into development of software or just modding software beyond the existing GUI.