I've done a little research work these past days, and I've stumbled across, the title might be a hint, free books!

The University Publisher of Kassel University offers books (dissertations, but other types as well) both as printed volumes and as pdf downloads, the latter being entirely free. You can see the whole programme of the publisher here.

Two books that I came across that may be interesting for you, too, are Presche, Christian: Kassel im Mittelalter and Wolkewitz, Ursula: Die gravierten Messinggrabplatten des 13. und 14.Jahrhunderts im Bereich der norddeutschen Hanse - ihre Herkunft und ihre Bedeutung.

There might be more, but unfortunately there is no good search function for the catalogue, and I have not had the time (nor the inclination or patience) to click my way through the long list. In case you find something interesting, too, please let the rest of us know in the comments!