[Radek]

1. After freeing at least 5 slaves, talk with some of the just freed slave. Talk about anything with him an you get the Twin Lamps topic.
2. After freeing at least 15 slaves, talk with any just freed slave about Twin Lamps. You get the "password" for Twin Lamps.
3. After freeing at least 30 slaves, talk with Jobasha about Twin Lamps. Jobasha shows out a sworn abolicionist and he gives you three stat raising books.

If you are a member of House Hlaalu and if (a) you have the "password" and (b) you are at least Kinsman then talk with Ilmeni Dren. You can do two Twin Lamps quests.
The first one (free Hides-His-Foots) can be done "on fly" even if you aren't Hlaalu. You do not get any reward but you do a good deed. The second one needs you to be Hlaalu (Ilmeni Dren must send you to the questgiver).