Nana, in case you speak Polish, I highly recommend you this book. It's a respectable scientific study (i.e. it contains no examples of the "cabinet mythology" that plagues many popular editions on Slavic mythology), yet it's written in a way that is accessible to wider auidiences (i.e. contains no "academic terminological stench" ). It doesn't deal with "lesser mythology" much and focuses on deities instead, but it still has a section dedicated to folklore demonology.