The Kronos Chronicles Series by Marie Rutkoski

Upon arriving in the Romany homeland to deliver the Mercator Globes, Petra Kronos, Tomik, and Neel formulate a plan to save Petra’s father, who has been transformed into a Gray Man. But when a long-held secret is revealed, Neel finds himself bound to his country. The friends are quickly swept up in an epic battle for power. Thrones are at stake. Spies are afoot. Murder is common. Worst of all, Prince Rodolfo is close to becoming emperor, and ruling half of Europe... moreUpon arriving in the Romany homeland to deliver the Mercator Globes, Petra Kronos, Tomik, and Neel formulate a plan to save Petra’s father, who has been transformed into a Gray Man. But when a long-held secret is revealed, Neel finds himself bound to his country. The friends are quickly swept up in an epic battle for power. Thrones are at stake. Spies are afoot. Murder is common. Worst of all, Prince Rodolfo is close to becoming emperor, and ruling half of Europe. How much is Petra willing to sacrifice to defend the people she loves? Her search for answers will take her to castles and cities, through mountains, and even underwater as she tests her strength and gambles her life. less