Sources on German spatial planning in English are scarce. It's my great pleasure to announce that "The Compendium of Urban and Regional Development" has just been released:

https://www.arl-international.com/knowledge/compendium

The (German) Academy for Territorial Development in the Leibniz Association (formerly known as The Academy for Spatial Research and Planning) (ARL) has been publishing the Handwörterbuch der Stadt- und Raumentwicklung for over five decades. And now a selection of entries has been released as "The Compendium of Urban and Regional Development".

The compendium is open access and part of the ARL's international outreach (https://www.arl-international.com/academy).