It happened multiple times to me too. Sometimes when I go past the mirror the traps are already firing in there and the book is gone, but I haven't been able to understand what causes it.
In one case I went past the mirror, then left to recover the key to the book and when I came back it was gone, with traps firing. I just figured I should have picked it up right away.
In another case I got past the mirror, saved and quit the game. When I resumed playing, traps were firing (they weren't when I saved).
Last time I was there I killed the undead in the caskets and when I went past the mirror, the traps were firing. I reloaded, ignored the caskets, went past the mirror, recovered the book and then dispatched the undead.
I'm pretty sure in none of these occasions I had a quazit with me.
When that happens the traps have knocked the book behind the altar. You can get it out by going turn based and moving the camera angle to see behind it.
