The Second Princess

Elodie was raised to be Queen of Corona.
She knows every royal decree by heart. She can translate Old Coronan alchemy notes, solve an equation before the Royal Mathematician finishes writing it, and recite banned histories from memory. She was perfect, prepared, and next in line.
Until her older sister came home.
When Rapunzel returns after eighteen years missing, Elodie goes from future queen to... second princess. No crown, no role, no idea what she's supposed to be now. So she does what she always does when she's anxious - she hides in the royal archives and overworks.
In the archives, she finds two things that don't belong.
One is a boy from Old Corona with black hair, blue eyes, and a lab that explodes twice a week. Varian is an alchemist, a nerd, and the only other person in the kingdom who obsesses over footnotes the way she does.
The other is a set of black rocks growing where they shouldn't, humming with a history the King swore was erased.
Corona wants Elodie to smile and be the spare princess.
Elodie would rather read the truth in the margins.
A story about second children, forbidden libraries, and the girl who was supposed to be queen - and decided to be the keeper instead.