One of the best books on this topic is called Mitologia słowiańska (eng: Slavic Mythology) by Aleksander Bruckner, but I'm afraid it's only available in Polish for now. There's also several books on Slavic folkore/culture that are ok, but don't contain as much about the actual pagan beliefs as the one written by Bruckner. Those include for instance Slavic Folklore: A Handbook by Natalie Kononenko and Slavic Traditions & Mythology by Stefan Cvetković.