Marykate's Diary is a book found in Dishonored 2.
The look on his face as she vanquished Emily the False and her Serkonan father will forever keep a candle flame flickering in my heart. Watching him beat a frantic retreat, knowing he'd return with others of his bloviating sect; knowing they would fall before us. It was delicious.
It can be found in a room on the third floor of Dunwall Tower in the mission Death to the Empress.