Franz Kafka – the workers' friend (2018)
marywcraig.com
marywcraig.com
https://en.wikipedia.org/wiki/Franz_Kafka:_The_Office_Writin...
(Also I believe these letters are 'anonymous' not in the sense that their authorship is unknown, but in that they were published under the institutional name of the Insurance Institute where he was employed, not under his personal name.)