The Wolf in the Cloister by Emily Leverett

Marie de France was born to be a princess and brought up to be a witch, but when her heart is broken, she takes refuge as a nun at Shaftesbury Abbey and leaves the familiar world behind. She might never love again or learn the truth about her Celtic mother’s magic, but at least she’ll have the comfort of her books.
Bleiz Clavret, lord of Sarum Castle, returned from the Crusades a changed man: bitter, broken, and cursed with a magical “gift” he never asked for and barely understands. Now infamous for debauchery, he hosts notorious parties attended in secret by the most powerful men and women in England; his name is known by all. But he hides his darkest secret from even his closest friends. Three days a week, he is literally a wolf.
A stolen relic and the rising of a demon bring Marie to Sarum Castle to search Bleiz’s impressive library for clues to the source of both. Intrigued by her sharp mind as much as her seemingly innocent beauty, Bleiz begins by trying to seduce his guest. But he soon discovers this “little nun” might well be more than his match. Equals in mind as well as courage, they discover a mutual attraction much too powerful to resist. But dark forces are gathering around them, and an evil from the past threatens to tear them apart.