I spent some quality time at a library today, and I found some great stuff. However, it was hard to narrow things down because the parts that were interesting and the parts that were pertinent were two different things. I only got through about half the books I found relevant to my research until I had to leave to do other things. I'll perhaps go back tomorrow or early next week.
What I may do on my next visit is instead of keeping to a specific era, I'll just mine my sources for interesting cultural tidbits and the like. After all, I'm doing this for a fictitious setting, so I don't have to worry about getting the specifics right.
For this setting, I'm particularly interested in the social structure. I learned some things about voivodes and kenezes, which was interesting since it gives a more solid way to present politics in the setting. I'll probably focus on pre-Christian beliefs and practices to flesh out the religious element. I'm also ever on the look-out for interesting customs, particularly births, rites of passage, marriages, funerals, and other religious rites.. Visually, I'm intrigued by the layout of settlements and the particular style they use. I'm drawn to the Gothic style, for obvious reasons, although I've learned the Slavs changed the style slightly to focus more on a unified space than the extremely vertical structure of most Gothic buildings.