Rhapsodic (The Bargainer Book 1) by Laura Thalassa

Callypso Lillis is a siren with a very big problem, one that stretches up her arm and far into her past.

For the last seven years Callie has been wearing a bracelet of black beads up her wrist, magical IOUs for favors she once received. Only death or repayment will fulfill her obligations.

Everyone knows that if you need a favor, you go to the Bargainer to make it happen. He's a man who can get you anything you want … at a price. And everyone knows that sooner or later he always collects. But for Callie, he's never asked for repayment. Not until now. 

When Callie finds the Bargainer in her room, a grin on his lips and a twinkle in his eye, she knows things are about to change. At first it's admitting a truth—a single bead's worth—acknowledging the attraction between them. But the Bargainer is after more than just rekindling their connection. Something is happening in the Otherworld. Fae warriors are going missing one by one, and only the women are returned, each in a glass casket, a child clutched to her breast. 

For the Bargainer to save his people, he'll need the help of the siren he spurned long ago. If she can forgive him.