I've found this from the Amalur wiki:http://amalur.wikia.com/wiki/The_Purloined_Letters
Don't know if this helps or not
I've done a little further digging and I found this video walkthrough on IGN's site:http://www.ign.com/wikis/kingdoms-of-amalur-reckoning/The_Purloined_Letters
this is where you are right now? In the game, I mean?
Go to around the 21.30-21.45 minute mark to see what I think is where you are in the game
Maybe this will help, too:
Once you grab the Missives of Sable, an alert will sound anyway - be prepared to sneak/fight your way back out (the exit is the lever-operated hidden wall at the northern-most area of the map). Leave by ladder to the Forum and go to the Hospitalis Quarters to meet Grim.
I also found this on on youtube:http://www.youtube.com/watch?v=zSeTMchmaMw
Please go the 11:30-11:45 minute mark to watch this solution.
As I see it, the trick is to light one brazier, then light not the second one, but the third, or maybe the fourth? one (from the first one, you've lighted). But all should be clear when you watch the video, I find
Hope this helps