[Shadowed Shards of Light]

Well if anyone here knows which guild this is in it's the Thieves Guild, I'm trying to finish it so I don't have to do any more Thieves Guilding. I got inside if the imperial palace and right when I got to the elder scrolls librairy, I broke my last lockpick trying to open the door. now I can't get back in because I need the guards room key to open the door to the guards room (or whatever the rooms named) the palace key doesn't open this door. So if anyone knows where I can get/steal the right key let me know. It shouldn't be a problem stealing it because my I'm a master at sneak so If anyone knows or has the guide book please answer back.