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
History of Schleswig=Holstein for the Foreign Office [British 1920]: https://tile.loc.gov/storage-services/service/gdc/gdclccn/a2/20/00/89/4/a22000894/a22000894.pdf