This thread will accumulate useful links that give helpful background information about people, places, events and whatever else comes up in the series.
Also, I've mentioned this in another thread, but the actual Book of Corrections written by Ryu Seongryong is available as a paperback book in an English translation. It was published by Cal-Berkley's Institute of Asian Studies in 2002. Go to bookfinder.com and you can get a used copy for under $20 or a new one for $24. I got a copy after IYSS ended and thoroughly enjoyed reading it. Excellent translation, and the text itself was very easy to read. Should be a great companion to watching this series.