It's all over the Archives floors. One is in the small room to the left of the very first room, on the floor in the corner, the other I think is near the dark-haired woman on the spikes, the third one is somewhere in the dark vertically long area where one shinobi-type enemy stands, to the left of the first library, and the fourth one I don't remember. But they're all in this location so if you interact with all floor tiles you should be good.
Spoiler below.
Also you can open a path to the secluded cave somewhere there if you interact with a book in the first library. I don't remember which one tho.