Matthias Hardt, ”Hesse, Elbe, Saale and the Frontiers of the Carolingian Empire”, sidorna 2019–232 i The Transformation of Frontiers: From Late Antiquity to the Carolingians (i serien Transformation of the Roman World, volym 10. Leiden 2021. ISBN 9789004476394. doi:https://doi.org/10.1163/9789004476394_013.