The Robin on the Oak Throne by K.A. Linde

The only thing worse than fearing a monster is falling for one…
Kierse McKenna just shattered the Monster Treaty. Again.

It wasn’t entirely her fault. The job was supposed to be simple: steal a goblin-made bracelet off of the Queen of the Nymphs in her own palace. Trade the bracelet for a way to uncover the truth about her past. Except everything goes sideways.
And then he shows up to save her.
Graves—the warlock who ensnared her, betrayed her, and left her to fend for herself. He’s a villain. A monster draped in charm and shadows. And gods help her, he always knows exactly what she wants.
But Graves never does anything for free. He has a job for his favorite little thief. One that will pit her against the most powerful monsters in existence, including his mortal enemy, the Oak King.
An ancient artifact has been located, and only together can they hope to steal it. She just has to let him in.
But once she lets a monster in, he’s impossible to forget…and even harder to resist.