Her Demon Bargain by Mags Boyd

In a city where magic is bureaucracy, bail bond agent Teddy Baptiste plays by the rules, until her brother vanishes. Her only lead is Azriel Callow, a three-hundred-year-old "sealer" bound by ironclad, supernatural oaths.

To find the truth, Teddy must enter a thirty-day contract with a being who treats every sentence as a binding agreement and has never once broken a promise. In a world of ink, wax, and ancient law, Teddy is about to discover that the most dangerous bargains aren't the ones made with the devil, but the ones you negotiate with your own heart. If she can survive the contract, she might just solve the case. If she fails, she'll be bound to a demon forever.