I know this is a much-debated topic in the scholarship, so I'm looking for some pointers on what to read to get the most up-to-date theories. Preferrably anything that's accessible online via Shibboleth since it's not possible to get to libraries right now, but if not that's fine too.
As for the theoretical side of seiðr study, I suppose you have somehow to access the works written in Scandinavian languages, like Brit Solli, Seid : myter, sjamanisme og kjønn i vikingenes tid, Oslo: Pax, 2002.
Neil Price, The Viking Way: Magic and Mind in Late Iron Age Scandinavia, 2nd ed., Oxford: Oxbow, 2019 is the latest work in English on the topic(, though I only have the 1st edition).
While these two works are written by archaeological anthropologists, historians and other scholars tackle this problem from a more conservative point of view, mainly based on written evidences:
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Reference (up-to-date state of research essay):