Gray Court by Hailey Edwards

After wiping out an entire black witch coven on the shores of Lake Pontchartrain, Rue can no longer afford to have a laissez-faire attitude toward the Maudit Grimoire. Its black magic is infecting her, and it’s leaking into her familiar bond with Colby, poisoning the loinnir with its thirst for violence.
A trip to the fae realm ought to be just the ticket, but any hope for a quick fix dies when a message arrives from High Priestess Naeema. Her daughter has gone missing, and she expects Rue to find her. As much as Rue wants to help Naeema, can she afford to put off disposing of the cunning book yet again?