At the base of THE OBSERVATORY, north of the district, is a man sitting and suffering. Talk to him to start the quest.
Follow the man and help him out by interacting with him when he suffers.
For the first page, the “Flying Pages” are sadly introduced… :-). There is a page on the roof of the small building nearby. Follow the page to catch it!
Flying Paper end point location on the map:
For the second page, climb up the Observatory to the roof. Here, on the table is the second page. Be aware that the page is not highlighted in the assassin’s scanner.
The third page is inside the OBSERVATORY. To get it, go back to the ground level. There is a barred door. Go to the back of the observatory. There is an open window, where you could throw a knife to destroy the barricade on the door, but it is too high up.
Notice, there is a movable shelf to the left. Move it closer to the window and climb it so you can throw a knife.
Enter the OBSERVATORY and get the page on the second floor.
When you come back, the Astronomer has passed away and his son is on his side. Talk to him to finish the quest.