The International Institute of Social History have recently put online a database of hundreds of Yiddish letters in the IISH archives (with a brief abstract of each). http://socialhistory.org/en/news/yiddish-letters-accessible
Historians of anarchism will be most interested in the material from
the archives of Boris Yelensky, Senya Flechine, Rudolf Rocker and
Alexander Berkman. Details are given at
You really need to download the database if you want to see the information, though there is some introductory material linked from this page.
Sadly, the International Institute of Social History have also announced he death of Dr Fuks-Mansfeld http://socialhistory.org/en/news/rena-fuks-mansfeld-passed-away