Source for all of this (details that are wrong are my fault):
It's a bit old -- 2004 -- but in English. Nothing about ebooks in it.
Obvs, in this system, you want to be the publisher AND the retailer, and German law is not particularly tough on verticals (which is why there was a Weltbild to go under last year). If you're thinking KDP in this context, you are not alone. You should also be thinking about AmazonCrossing.
I'm less sure how the Bonnier/Amazon relationship is impacted by fixed pricing. If Amazon is importing Bonnier books into Germany for sale, then Amazon gets to set the price (Bonnier isn't German; it is Swedish. I don't know which part of Bonnier is currently entangled with Amazon). But if the negotiation involves a Bonnier-owned German company (of which there are several), then Bonnier is deciding pricing AND cut for each participant in the chain. Bonnier is _really interesting_. First off, it is operating in a lot of languages in a lot of countries, and some of those countries _had_ fixed price agreements and some of them no longer do, and some of them got rid of the agreements and then brought them back. And it's a family operation, if wikipedia is to be believed. Among other things, they funded the super awesome app maker, Toca Boca.
I would like to point something out about the German book distribution system, which is directly related to the pricing controls. There is a dense network of retailers of books, but they cannot stock everything. There is a widespread network of distributors, and the claim made in the 2004 summary above is that even in the countryside, at a tiny book dealer, you could get any book you wanted within 24 hours. This would be accomplished by going to the bookstore, the bookstore getting the book from the distributor, and then returning to the bookstore to pick it up after the bookstore got it. This is _exactly_ the model which Amazon has re-adopted while negotiating with Hachette and Bonnier. This is in fact the original model that Amazon operated under, back when I was one of its early employees.