Assuming the chains are still lowered (so you triggered the platforms in front of the hallway to the exit, jumped up and hit the button to lower the chains, but did not pick anything from the 4 nearby chains), head back to other end (where you started, if you used the scrying stone yourself). The skill book is on a chain in a corner, IIRC on the left behind a pillar/wall from where you lowered the chains, and out of a direct line of sight. You can see the skill book on the chain from that part of the room if you look up before it is lowered, but from the platforms there is little or no indication there is a fifth chain (don't recall if you can see a bit of chain, or not).
