Pre-Christian Slavic and Baltic religion is worse attested than almost anything else this atlas covers, and honesty about that matters more than a full-looking page.
There is no native text. The written notices are brief, external and mostly hostile: chronicle entries recording the destruction of idols, a handful of passages in medieval Latin and Old Church Slavonic works, and descriptions by German and Danish writers of the temple sites on the Baltic coast that they or their allies were in the process of destroying. A list of divine names survives; what was actually believed about most of them does not.
Archaeology supplies sanctuary sites, carved posts and deposits. Folklore collected in the nineteenth century preserves calendar customs, songs and figures that plainly predate Christianity, but it was collected eight hundred years after conversion and had passed through those centuries.
Into that gap a great deal was written. The Romantic and national movements of the nineteenth and twentieth centuries wanted a pagan past and produced one, and some of the material is outright forgery. The Book of Veles, presented in the twentieth century as an ancient Slavic scripture, is rejected by philologists on linguistic grounds and continues to circulate.
The result is that a confident-sounding account of Slavic mythology is usually reconstruction, and the reader is entitled to know which parts rest on a chronicle notice and which on inference.