Vespertine

Artemisia is training to be a Gray Sister, tasked with cleansing the bodies of the dead so that their souls can pass on; otherwise, they will rise as spirits with a hunger for the living.
When her convent is attacked, Artemisia defends it by awakening a revenant. Wielding this ancient spirit's extraordinary power almost consumes her - but only a vespertine, a priestess trained to wield a revenant, has any chance of saving Loraille.

As Artemisia unravels a sinister mystery of saints, secrets and dark magic, her bond with the revenant grows. And when a hidden evil begins to surface, she discovers that facing this enemy may require her to betray everything she's been taught to believe - if the revenant doesn't betray her first.

Praise for Sorcery of Thorns:
'A bewitching gem . . . I absolutely loved every moment of this story' - Stephanie Garber, #1 New York Times bestselling author of the Caraval series
'If you loved the Hogwarts Library . . . you'll be right at home at Summershall' - Katherine Arden, New York Times bestselling author of The Bear and the Nightingale
'An enthralling adventure replete with spellbinding characters, a slow-burning love story, and a world worth staying lost in' - Kirkus Reviews, starred review