A few days ago the Frankfurter Allgemeine Zeitung informed that Hartmann’s unpublished documents (letters, manuscripts, discussion-protocols, etc.) reached the Marbach Archive (this is Germany’s most important archive for the legacy of authors and thinkers, especially of the 20th century).

The following article (in German) adds some information: Deutsches Literatur Archiv

Another good (German) article is: hr-Online