The Duke and the Disciple is an age gap, teacher/student romance, featuring Duke Rahn and Aesylt, characters first introduced in The Claw and the Crowned. It is the second story in the Sceptre Cycle of The Book of All Things, but can also be enjoyed as a standalone.
The Sceptre Cycle:
Stolen thrones, love triangles, arranged marriages, and fractured duty from the descendants of Ilynglass (high heat)
The Claw and the Crowned
The Duke and the Disciple