Reddit reviews Creatures of Slavic Myth (The Slavic Way) (Volume 4)
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Most of it is Slavic folklore and mythology.
Amazon has one English book especially about Slavic mythic creatures (don't own, don't know if it's good), you should find a lot of the monsters of The Witcher Series in it, though sometimes with a slightly different name.
Some other things, like the motive of the Wild Hunt are things that are elements in most European countries folklore. Wiki is a good start for things like that, because you find good sources on most articles.