Blood on the Bark by Emily Hickey

When a dead druid is found tangled in the branches of the World Tree, Investigator Hian is sent to uncover the truth. The case should be simple—until he discovers his unwanted partner is Kaela, a fiery shapeshifter from the brutish Fauna Tribe and the woman he once loved… and lost.

As Hian and Kaela chase the threads of the murder, they uncover a conspiracy that threatens those of Hian's Flora Tribe and could destroy everything their people were created to protect. But the closer they get to the truth—and to each other—the more they risk being consumed by it.