Slavic paganism
Sources about Slavs and their beliefs mainly come from the old Ruthenian chronicles and notes of German bishops. A lot of information contains Helmold's Chronica Sclavorum. There's also Chronicle of Thietmar, and the other chronicles, although they are not the only sources of information about Slavs and their beliefs. There's also for example from the diary of a Jewish trader of the Slaves - Ibrahim Ibn Jacob.