Synthesized answer
The provided passages do not contain a list of unanswered questions. However, the text implies that certain questions remain to be answered through the course of the investigation.
Specifically, it is not yet clear what incendiary information is contained within the missing letter from the Emperor to his mistress [Passage 1]. Additionally, the nature of the connection between the death of Herr Karl and the missing letter is not fully revealed [Passage 1]. The passages also do not state what questions remain unanswered by the end of the narrative.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: The Third place by J. Sydney Jones Description: March, 1902. When Herr Karl, head waiter of the Café Burg, is found dead at the foot of the Maria Theresa monument, it is assumed he slipped on the ice and hit his head. However, a witness has come forward who says otherwise, and private enquiries agent Karl Werthen is hired to investigate. At the same time, Werthen is commissioned to locate a missing letter from the Emperor to his mistress. Franz Josef is desperate for the letter not to fall into the wrong hands - but what incendiary information does it contain? As Werthen and his…