An inventory of sources on history of Schleswig-Holstein: link to pdf
Duke and Princes: https://dukesandprinces.org/2020/12/21/dukes-of-oldenburg-and-schleswig-holstein/
Innovative Feudalism: https://www.jstor.org/stable/41330066
The Wadden Sea Region: https://www.jstor.org/stable/j.ctv7xbrmk