OK, I am in the same place. I read the goblin's mind, found the book, etc. That part was straightforward. I also see that my minimap wants me to find a book in the goblin's secret room.
I see a small locked chest on a table, but I don't have a key to open it. Where do I get the key for it?
--EDIT--
OK, I see now:
There are more floors in this place. The key for the box drops from an undead laying on a table to the left side of the 3rd floor