Hey, this is JT, the English translator for the game. I'll try and help out with questions about the game where I can.
On Floor 10 of the library, count the third shelf from the bottom of the area, then go to the third shelf counting from the left wall and interact with the shelf to get an Old Spellbook.
Then, go into the room that is second from the bottom of the area (the one above the door to go to Floor 11) and interact with the shelf in the center to open the passage to the bandit lair.
Are you referring to a gallery scene? It should unlock after fighting him.