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This is a good start on the sources. You probably don't need: Muhammed, Murtala Ramat. Scientific Results of the Viking Project. He was a military leader, and you can probably get a better history of the area.
This is good on articles, but could do better on books. The Imperato is an exhibition review. Can you get the catalogue for the actual exhibition?
Also, look up Herbert Cole’s book on Southeast Nigeria. Consider looking at Imo Nse Imeh’s book.