Where is that poison creature book.That is the last quest I have Than I can move on.
It is on the first level of the game, in the western spider cage in the arena (with the pit you had to jump down). Head back to where you entered the citadel and take the stairs down to get back to that level.
If you blocked the cogwheel earlier rather than fighting the spiders, there is a key in the medic's room which will allow you to open the cage doors.