History - 131 free documentaries about mongols & steppe empires.
Start with Mongolia: Rise and fall of an empire (52 min, DW Documentary), then Who was Genghis Khan? (44 min, National Geographic). All 131 documentaries here are free to watch from their makers' official uploads. To read deeper, the top-rated book on mongols & steppe empires is Genghis Khan and the Making of the Modern World by Jack Weatherford.