Fine overview of Viking Age Poland, in particular the ninth and tenth centuries. And the connections with the surrounding countries and kingdoms. The full reference is:

Mateusz Bogucki, ‘Intercultural Relations of the Inhabitants of Polish Territory in the 9th and 10th centuries,’ in: The Past Societies: Polish Lands from the First Evidence of Human Presence to the Early Middle Ages, edited by Przemysław Urbańczyk (Warsaw: Institute of Archaeology and Ethnology, Polish Academy of Sciences, 2016), pp. 223-276.