Did you mindread him earlier? I thought you had to do so to get Gothe's ballads to show up in the bookcase... If you didn't mindread him you should be able to do so during the conversation (hit X in the Xbox version; don't know for PC).
The wall beside that bookcase is movable, and at least after mindreading Erlking, reading Gothe's ballads removes it to reveal a small room containing a couple chests (one with the soul forge contract, the key for it being up a level). Maybe if you didn't mindread Erlking, the 'click inside the wall' is just a clue to tell you that you are in the right spot, and there is something there hidden.