Group Policy is one of the most powerful features of Windows Server 2008 and to fully understand it requires a good resource. The good news is that since the first publications on Policy settings in Windows NT came out, the resources have gotten better.
The latest addition to this family from Microsoft Press is the Windows Group Policy Resource Kit for Windows Server 2008 and Windows Vista. You will notice with Windows Server 2008 a number of the resouce kit books that came bundled in the huge Windows Server 2003 Resource Kit are now broken out and sold individually.
This new policy does not detract from the coverage. This is still a comprehensive resource kit useful to anyone, new or veteran to Group Policy.
UK Site: Windows Group Policy Resource Kit: Windows Server 2008 and Windows Vista
US Site: Windows Group Policy Resource Kit: Windows Server 2008 and Windows Vista
The Kit covers all the new areas you'd expect, using the GPMC from a basic level through advance management, modeling and reporting, complete details on Group Policy Preferences as wells as templates both the original format and the new ADMX templates. For those new to the Group Policy world there is a recap of the Group Policy environment, their architecture and delgation techniques. As with all resource kits, there are plenty of settings listing with description, steps to complete tasks and screenshots of the Group Policy world.
I said this book is for everyone who works with Group Policy and I believe it certainly is a great reference for feature. I've used it as the basis for an up and coming TechNet Event session for July 2008.