[Gem Mountain - North]
It's true. The bookcase apparently moves somehow. Rein expertly noticed this earlier.
There's the scrapes on the ground by the bookcases on the west side, which suggests that particular bookcase moves frequently. (The bookcases, of course, attached to the walls so it's not like you can just lift them up and move them.)
However, noticing the bookcase's floor-scrapes isn't all that obvious due to one simple reason: it's not the only thing that appears to have frequently moved. The nearby desk has also frequently moved up next to the bookcase and back several times. And by how deliberately the desk's floor-scrapes are, always moving to a similar spot right next to the bookcase, it suggests there's something more than somebody who just liked rearranging the furniture. Especially since the desk and the bookcase are probably the only furniture they've seen so far that appears to have frequently moved.
However however however.
Looking at the floor won't reveal any additional clues. There's no apparent trapdoors, pressure plates, buttons, or anything else hidden in the floor.
Looking at the walls doesn't seem to help either. None of the books scream "pull me" or look out-of-place. There's nothing else nearby those bookcases on the walls that looks like it could be used to trigger a secret passageway.
However, ((if you think you've got it figured out)),
On the ceiling next to the bookcase, there's a slightly discolored (due to age and weathering) spot which looks like it could act as a button. Whoever moved the desk would apparently move it so they could stand on it and reach the ceiling.
Press it, and the bookcase revolves completely around, only allowing a brief moment to go through the bookcase before it closes again.
However, if the desk is moved to underneath the button, the bookcase spins until it hits the desk and...stays open until the desk is moved, allowing some light to filter into the passageway.