I adore Firebird. That was one of my first books ever set in anything but a western european-style setting. I've not often found others that I liked, but I loved the entire world and how believable it was and authentic it felt, despite my complete lack of any background knowledge in Russian culture. I don't know anything about the Five-Hundred Kingdom books, though. I've read mostly what my parents had, and we don't have any of those.