The trilogy covers hint at a main plot point/twist in the story
This trilogy was so addicting that I powered through reading them all within a week lol. Anyway, after admiring the covers, I realized that the design wasn't random at all, because before reading them i was confused why the crowns were in different themes.
In CP there are branches and the crown because Oak crowns Cardan in the end (quite witty and funny I would say).
In WK the crown is underwater because Jude got kidnapped by the undersea (which was a major twist given the other twists that made her end up there)
And in QoN there is ice, flower petals, the crown is broken, and a serpent because of the location of the court of termites, Jude healing herself as queen, then Cardan broke the crown which made him transform.
Idk if this is/was common knowledge but I just loved it when the realization hit me, which made me appreciate this amazing work of art more ♡