Stephen D. Rogers is the author of SHOT TO DEATH, THREE-MINUTE MYSTERIES, and and more than 700 shorter pieces. His website, www.StephenDRogers.com, includes a list of new and upcoming titles as well as other timely information.
Hi Stephen - alas for your question about German mysteries with private investigators before 1941 - I know of none. You migh direct your question to Christian Koch from the Berlin Mystery Bookshop HAMMETT:
hammettkrimis-c.koch at snafu dot de
Good luck with you research!