I just started reading Elantris by Brandon Sanderson, and Sarene immediately reminded me of Eilonwy from The Chronicles of Prydain - similar looks, personality, and glowing orb companion.
During episode 4, when the characters all go on a ski trip, it's mentioned a couple of times that Stella is reading a book. You can sort of see the cover of the book at 7:30 (Stella holding the book) and at 25:53 (Kitty looking in Stella's suitcase). Sorry I can't attach screenshots because Netflix won't let me. But from those glimpses and the size of the book, I recognized that it was the clothbound Penguin classic edition of The Count of Monte Cristo by Alexandre Dumas!
This in itself is maybe not that interesting, but the themes of this book and the fact that Stella was reading it was pretty interesting to me (major spoilers after this point in case you haven't finished the season yet!).
The Count of Monte Cristo is famously a vengeance story, and as we discover by the end of the season, vengeance was Stella's goal. I just thought this was a fun and subtle clue included to hint at what Stella was actually planning before anything major got revealed about her plans and intentions!