I was delighted to find that Herbert Birett’s site Quellen zur Filmgeschichte und einiges anderes is still active. I lost track of it some years ago, when it seemed to have taken down, but it is alive and well under the more memorable web address www.kinematographie.de. Birett is an assiduous chronicler of German film history, whose speciality is filmographies based on censorship files and other forms of official records. His major work is the book Das Filmangebot in Deutschland, a listing of 17,000 films shown in Germany 1895-1911, which he sells for €70, but there is much of his site that he makes freely available.
There is a title list for all German films 1921-1930; Weimer Republic censorship records 1922-1932; a bibiliography of German film monographs to 1914; a listing of German movie journals to 1920s with notes as to their location in libraries; plus other resources for German sound films. It’s in German, but with some helpful short descriptions in English for each section. Birett’s work is exemplary, and the site is a key resource for the specialist.