I can recommend the book "Royals and the Reich. The Princes von Hessen in Nazi Germany" by Jonathan Petropoulos (who also wrote excellent books about the art world in NS Germany). The book covers far more ground than the title suggests but it also gives a fair assessment of the involvement of the house of Hessen in NS politics. Hitler used the well-connected high aristocracy to acquire a veneer of respectability but once he didn't need them any more, dropped the princes like hot potatoes. https://www.amazon.de/Roy...sen-Germany/dp/0195339274
The dynamics between the six Hessen princes and their parents is described, and especially Christoph and Philipp turn into real persons for the reader. I'm a big fan of their liberal, intelligent grandmother, Empress Frederick, and found it horrifying to read how from that heritage, her grandsons (some of them) turned into Nazis. The author shows how that happened.
The house Hesse is closely and in many ways related to the more famous and powerful houses of Windsor and Romanov (and all the others...), its members are cultivated and highly educated and anything but provincial. Very interesting.