Shadow of the Crescent — Margaret Ball

Twenty-seven years after the fall of Constantinople... Caterina, Countess of San Florian, keeps a book of important things she's learned, from poison antidotes to cosmetic recipes, from charms for toothache to ways of raising and commanding demons. Having a reasonable care for her soul, she has never actually tried demonic magic. Yet. Gian, captain of her personal guard, has an innate magical talent that does not rely on incantations, but warns him of danger and awakens him to opportunities. It makes him nervous. And Sultan Mehmed II wants one last great victory. San Florian would be an excellent base of operations for his army to attack Venice. On the run from Venice to Constantinople after the Turkish-aided takeover of San Florian, Gian and Caterina will need all their wits and every scrap of magic they can employ to escape, to survive, and to recapture their city.