The Gods of the Ancient Slavs by Myroslava T. Znayenko

This is a great book on the old Slavic religion, it's an important, scholarly work that is available online for free from Slavica:

https://slavica.indiana.edu/system/tdf/bookContent_pdf/08_SLAVICA%20REISSUE_Gods%20of%20the%20Ancient%20Slavs.pdf?file=1&type=node&id=634&force=