Beginning with the first book, The Bear and the Nightingale, this beautiful fantasy trilogy is set in a version of medieval Russia in which history and myth coexist. In Witch yet more characters from Russian folklore are woven in as fiery heroine, Vasya, faces the foes of Moscow, Rus', and humanity... whether the powers that be are behind her or not.
I recommend reading this dark, mystical book on a chilly night (forecast looks promising) — but only after finishing the first two in the trilogy!