Having trouble finding the Lexicon? I did too, here is a video showing exactly how to get there!
Use Clairvoyance to get to the door that leads out this way, once you get out there head to the arrows on the left (skipped this in the video, but go ahead and do it) and they will hit that robot lady (rosy? jk). After that you can fight the farmlers and other creatures or you can just run like a little bitch like I did. It really doesn't matter. You're destination is the "Tower of Mzark", you pull a lever to get there. After that go ahead and put the cube down and press the button (video shows exactly how to press them)