From watching the stream I saw Gandreas list this book as an inspiration for the plot of Cythera (along with a minor in history).
Here's the wiki for the book:
https://en.wikipedia.org/wiki/The_Riddle-Master_of_Hed
"Each ruler has a mystical awareness of his or her land: the land-rule. The seldom-seen High One presides over all."
Definitely qualities of the Land King.
The plot summary says that the main villains are shape shifters and that one may even be posing as the high one. Which sounds a lot like the Undine and the plot of the game's antagonists.
Additionally, the world map for the books looks similar:

I didn't find much else looking at the summary of the two sequels but if anyone has read these books or plans too I would love to hear any more insights.