Battle Magic by Tamora Pierce
Tamora Pierce is a young adult fantasy writer known for her strong, usually female characters. This book follows after Street Magic, and is set before Melting Stones in the Circle Opens series. Briar Moss is a young plant mage, traveling with his mentor Rosethorn and his student Evvy, a stone mage. They are welcome in Gyongxe, which feels like a version of Tibet, where they meet the young god-king, and travel in the mountains. Rosethorn is invited to visit the Emperor Weishu in neighboring Yanjing, and tour his magnificent gardens. They quickly learn that he is cruel and greedy, and rescue one of his captives. Traveling with a caravan out of Yanjing, they learn that Gyongxe is to be attacked by Weishu’s armies, and feel compelled to help. Along the way, Evvy is captured and takes refuge with Luvo, a mountain deity. Rosethorn and Briar Moss are startled by small gods coming to life to protect their mountain home, and they learn how awful war can be, especially when they have to choose between using their powers to heal or helping win the battle. This is a darker book then many by the author, and I would suggest starting with Street Magic, or even earlier with the Magic Circle books.