So, I'm planning to run
But now I find that I know next to nothing about the AGS. How did they come to be? What happened to Berlin? (Neo-Anarchist paradise?)

There is German Source book with all the details for Germany. However, the adventure you want to run isn't based on the source book and really has next to nothing to do with germany, current or canon SR IMHO. In fact I scrapped that run altogether. I had the item in question transferred to a museum for exhibition in Seattle and the Runners had to catch it on the transfer or in the museum.

In what sourcebook do I find this adventure?

The adventure Jong is referring to is one of a series that make up the 1st Edition campaign book Harlequin. It involves a brief trip to Bavaria IIRC and has very little to do with the contents of the Germany Sb.

The Germany presented in this adventure was a helluva lot more fun than the dry textbook that was the Germany Sourcebook. I miss laubenstein's artwork; it brought the whole magical part of Shadowrun to life. That and Bradstreet's work before he became uberfamous. Ugghhh...nostalgia....taking over....must...go...on...

... and where the only law is the requirement to take down anyone or anything which looks as though it is beginning to make laws over others.

You might like this take.