Demonic Triangle (Doomed Cases Book 1) by Joanna Mazurkiewicz

A missing royal. A love triangle. Can Maxine crack the case before her heart is ripped in two?
Maxine Brodeur has spent the past year recovering from a devastating breakup with Prince Arthur — the realm’s future king. She’s kept a low profile as a half-human/half-demon, spending her days running “Doomed Cases,” a private supernatural agency. After illegally revealing her powers to save a human girl from attack, she gets an ominous letter bearing the royal seal.