The Wren in the Holly Library

Thirteen years after monsters emerged from the shadows and set off a cataclysmic war, New York lives under the Monster Treaty. Kierse McKenna is a thief with a found family and a flexible sense of right and wrong.

When a job to steal a diamond ring from the monster Graves goes wrong, she breaks the treaty and ends up bound to a deal with him: help him steal a magical artefact from the King of the Monster Underworld. A rival gang of Druids is pressing in as well, and Kierse has to decide who she can trust.

The first book of the Oak and Holly Cycle, a romantasy series.