Changeling by Philippa Gregory

Set in Italy during the year 1453, this novel follows the adventure of four main characters and shows their struggle with life and religion. With the help of his friend Freize, Luca uses his logic to solve mysteries that can only be explained by others as magic or the supernatural. Meanwhile, Isolde and Ishraq are trapped inside a nunnery where strange things are happening. It's inevitable that their paths cross.