Young Adult Fantasy
Grishaverse - particularly the original
Shadow and Bone trilogy. The magic system--the Small Science--fascinated me, and plays heavily into the scientifically-studied
Power Carriers of Gormhan.
Red Queen series - honestly some elements were a strong rebellion against the "anyone can betray anyone" mantra and some play right in...
Seraphina - facinatingly worldbuilt dragons at the palace, and a religion focused on
Saints. The Grishaverse's Saints also play into mine.
Classics
The Age of Innocence - Old Money New York and the inability to change for someone entrenched in the system, even if he wants to escape... the foundation of my nobility.
Macbeth - the classic in scheming nobles with blood that won't wash away.
Henry IV Part 1 - a prince with two faces: one for his fellow nobles, one for the commoners of the pub he frequents.
Younger YA & Middle Grade
Gallagher Girls series - the titular Gallagher Academy is a strong inspiration for
Everdon Academy, though Everdon is for young nobles rather than young spies. The difference does become blurred at times...
Dragon Slippers trilogy - my favorite dragon books growing up.
Tales of the Frog Princess series - another set of wonderful middle grade books about dragons and a princess heroine with magical powers.