Full video walkthrough with English commentary!
Inside the Vatican Post Office, we can find Catherine's Letter on one of the tables, which starts the fieldwork The Mad Priest.
Head to the Sistine Chapel to meet with the nun who wrote the letter. After meeting her, return to Antonio in his office at the library for another conversation and to receive the Sewer Key.
The Tower of Nicholas is a circular building past the checkpoint outside the Vatican Post Office, so go there and head downstairs to progress through the sewers. We will need to do some climbing, jumping, and swinging to reach the top and enter the Palace through a side room. To the right of where we enter is an unlocked chest with the book Sister Catherine requested.
Bring the book back to her in the Sistine Chapel to complete this fieldwork!