Early Medieval Slavic mythology

It's a bit of a shot in the dark, especially given that it's an outsider's perspective, but I would suggest reading Thietmar of Merseburg's Chronicon which is a very detailed record of life on the Imperial German frontier in the late 10th and early 11th centuries. As part of his chronicle, Thietmar discusses the political machinations between the Emperor Otto II and various Slavic neighbours, enemies and allies. He also includes a surprisingly in-depth and seemingly unbiased record of the holy sites and ritual practices of the Wendish people.

I'm afraid there is not much on the topic of Eastern European mythology in any language, mainly because Slavs themselves did not know writing (despite some pan-Slavistic pseudo-historians claiming that this is a stereotype created by Germans in 19th century to present Geermanic nation as 'culture bearers'). Scarcity of foreign records also makes it difficult to reconstruct even such simple elements as type of the pre-Christian Western Slavic beliefs (it is still unsure whether they utilised a pantheon in the Nordic or Mediterranean sense or were more polydemonic in nature, as evidenced by folk rituals recorded by ethnologists since early 19th century.

Thietmar's 'Chronicon' mentioned by u/BRIStoneman is a good start (along with ' Res gestae saxonicae' by Widukind of Corvey), although you must take into account the fact that it focuses on Rugians who could have been influenced by their Germanic or Norse neighbours and thus may not be representative to Slavs in general, even for the Obordrites Union they bordered with. Another good read, focusing more on the eastern part of Europe is Nestor's chronicle, commonly referred to as 'Primary Chronicle' or 'Tale of Bygone Years' that is a good contemporary source of the history of Kievan Rus (and its Slavic-Norse origins, although keep in mind that Nestor was a chronicler during the reign of strongly pro-Scandinavian Svyatopolk I and this bias could have influenced the description of historic events). Keeping up with the Christian medieval authors, it is worth to mention 'Chronica' by Cosma of Prague. Both Cosma and Nestor mention some elements of pre-Christian Slavic beliefs that despite living in 12th century they could have witnessed first-handedly as they mention explicitly that pagan beliefs are still alive in some regions, if rare. For example, they describe practice of building small buildings or shacks by the roads for the souls of the departed ('as if for the well-being of souls') as well as cremation that in Christian lands has been universally replaced by inhumation.

If you're interested in earlier history, you may check Jordanes' 'Getica' that provides some information about the Slavs and their interactions with the Empire. Another sources are 'On Wars' ('Hiper ton polemon logoi') and 'On constructions' ('Peri ktismaton'), known in English as 'Wars of Justinian' and 'Buildings of Justinian' respectively) by Procopius of Caesarea who gives some insights into the early organization and customs of Slavs including probably the earliest information about Slavic religion. This should be taken with a grain of salt though, as even though his remarks of 'lightning-wielding deity worshiped by Sklavenoi' are quite often uncritically construed as an evidence of cult of Perun (Perunas, Perkunas), alleged Slavic counterpart to Nordic Thor or Gaulish Taranis, in other books Procopius claims that inhabitants of Thule worship Ares even though no land assumed to be Thule (e.g. Ireland, Iceland, Orkneys, Saaremaa) was ever subjected to Roman rule, what suggests that the aforementioned deity could have been a simple transposition of Zeus/Jupiter. This can, of course, suggest that Slavs had a pantheon presided over by a chief deity. As you can see, there is a lot of suppositions and guesswork even though I'm speaking about one of the best sources depicting pre-Christian Slavs.

Slavs are also depicted in the books written in the time of reign of Maurikios (Maurice) and Heraclius, namely in anonymous work 'Strategikon' (often attributed to Emperor Maurice) and Theophylact Simocatta's 'Chronologia'. It is interesting that while the first work and parts of the second one depict Slavs as competent, cunning warriors harassing borders of the Empire or mercenaries in Imperial army (quite consistent with aforementioned accounts of Procopius), the second one includes a story about three Slavs (Sklabenoi, a term used to denote Southwestern Slavs, in opposition to Antes and Wends, that is Eastern and Western Slavs respectively) who claimed to be envoys to Avarian Khagan and said that they are unarmed and carry musical instruments because they come from the lands where iron and warfare is unknown and thus they are accustomed to a pastoral, peaceful life. An anecdotal, yet interesting information.

Majority of sources I read on that topic are written in Polish, Russian or Czech. Here are three English sources I found in my notes:

Florin Curta, 'The Making of the Slavs. History and Archaeology of the Lower Danube Region, c. 500–700', Cambridge 2001

Speros Vryonis, 'The Evolution of Slavic Society and the Slavic Invasions in Greece.The First Major Slavic Attack on Thessaloniki, A.D. 597' in: „The Journal of the American School of Classical Studies at Athens”, 1981, issue 50, pp. 378–390.

Samuel H. Cross, 'Primitive Civilization of the Eastern Slavs' in: „American Slavic and East European Review”, 1946, issue 5, pp. 51–87 (quite dated but still worth a reading).