Hale Shannon Hale 
River Secrets [EPUB ebook] 

War between Bayern and Tira is finally over. To cement the peace with their old sworn enemies, a group from each kingdom will cross to the other for a 'season of friendship’. At first all is well, but mysterious events in the Tiran capital arouse suspicions and anger bubbles just beneath the surface. Enna’s friend Razo must find out who is masterminding these events before it’s too late and they find themselves trapped in the heart of Tira as war breaks out.