The Wrath And The Dawn by Renee Ahdieh

Every dawn brings horror to a different family in a land ruled by a murderous boy-king. But all is not what it seems in Khorasan, and it will take the wit and will of one girl to break the cycle . . . and the spell. The Caliph is a killer. Every night the eighteen-year-old ruler takes a new teenage bride and every dawn he has her killed. When sixteen-year-old Shahrzad volunteers to marry Khalid, the young Caliph of Khorasan, she does so with the plan of exacting revenge on him for the murder of her best friend. Shazi’s wits and will get her through to the dawn that no others have seen, but with a catch even she cannot outwit–she’s falling in love with the very boy who killed her dearest friend.