I've also really enjoyed Luke McCallin's trilogy about an ex-Berlin detective serving as an intelligent officer in German-occupied Sarajevo during WWII and William Ryan's trilogy about a Moscow detective set during Stalin's Great Terror. Any similar recommendations?
David Downing's "Station" series is excellent imo. Although not detective novels as such, most fans of BG should get on very well with them.
The obvious writer is Alanm Furst who has written several novels about spying pre and during WW2. His early novels like Night Soldiers, Dark Star, The Polish Officer etc are excellent. His later novels, with a few exceptions have been pretty poor, especially his last couple. Each novel has a different hero but the heoine of almost every novel is the city, Pari, about which he writes atmospherically. More than worth looking into.
Volker Kutscher's books about detective Gereon Rath are a good read. Set in Wiemer Germany. Babylon Berlin is very rich in the background of the times.