Book Obtained
I guess I forgot to mention that the golden key was magical. *Was*, because after Brogg opens up the case and takes the Book of Familiar Places, both the key and the lock cease to be magical. You now have the book.
Tut, there's nothing necessarily keeping you here. You can proceed up the stairs to either the Museum (level 6), to the Living Quarters, actually, levels 7-9, or all the way up to the Solar Observatory (level 10) Or head back to the elevator and go up to the Observatory the back way.