Group Policy: The technical editor's view

Okay, I have something to admit. I am the technical editor of Microsoft's Group Policy UA content, but...I'm not the World's Leading Expert® in Group Policy. I know—shocking.

If you were going to recommend content for me to study—white papers, guides, books, blogs, virtual labs, and so on—to improve my standing in the realm of Group Policy expertise, what would you recommend I study? I know this may be heresy, but I will say that you needn't confine your responses to Microsoft content only. But feel free to send unsolicited praise if you have it. :)

What are your favorite tools for learning Group Policy? Do tell.

CraigLi