Queen of the Tearling by Erika Johansen
Centuries in the future after the cataclysm of the Crossing, humanity has returned to something like a feudal system, living exclusively on the last livable continent with no modern technology to speak of. Princess Kelsea Raleigh Glynn is turning 19, the age of Ascension, which means she will soon take her mother's throne, but not if the ever-living sorceress, the Red Queen of the neighboring kingdom, has her way
Review from Publishers Weekly:
Johansen makes an impressive debut with this ambitious fantasy adventure, which takes place several centuries from now following the collapse of civilization and mass migration to a newly discovered continent. The resultant society resembles medieval Europe, with modern technology all but forgotten, and magic is subtly present. Finally come of age, 19-year-old Kelsea Glynn must evade assassins and her uncle’s nefarious plans in order to take her place as rightful queen of the nation known as the Tearling. Her first acts as ruler break a treaty with neighboring Mortmense and anger the unaging sorceress called the Red Queen; Kelsea rallies her allies and prepares for war. Johansen starts strongly, with a forceful, memorable heroine immediately thrust into a series of intense situations and forced to make dynamic, if overly idealistic, decisions. While the setting and backstory could stand further explanation and exploration, and many elements fall apart under closer scrutiny, this trilogy launch is still an engaging page-turner.