Books that gives You the most information on Perun, Mokosz, Triglav and ancient Slavic culture.
Pre-war ethnographic books are sometimes very informative because of customs, songs and festivals that ur ancestors did. Some had deeper meaning that You'd thought.
Sometimes these unique, often crazy attempts give You a nudge in the right direction and You make a new discovery on Slavic Shamanism and ancient religion. I respect these books as very brave.
Shamanism in Poland? Celtic and Greek festivals that You can still find celebrated in modern Slavic countries? You will find a lot of cool info in these well researched books.