Citadel of the Sky by Chrysoula Tzavelas

Her (not very) Serene Highness Princess Tiana tries her best not to think about the dark lords that regularly attack her country or how the magic in her bloodline makes her family go mad. The descendant of a legendary hero, she prefers the escapism of books and theater, not the battlefield.

Then a rash of suspicious deaths strikes the Regents—trusted advisors, friends, and guides to her troubled royal family—and the Noble’s Council tries to cover it all up. In order to protect her dreams of normalcy, Tiana is determined to get to the bottom of the murders and the conspiracy, even if that means making a dangerous pact with a telepathic demon trapped in a magical sword. But he may just be the edge she needs to save the people she loves…