The Dragon Egg Princess by Ellen Oh

Wow, this was amazing. This is by the same author as Spirit Hunters. Five years ago, Princess Koko of Joson disappeared into the Kidahara—the magical and dangerous forest that occupies most of the kingdom—and never came back. Present day, men have come to Joson and are planning to cut down the Kidahara. The king’s brother has come and taken over the kingdom. A boy named Jiho has to join them so he can pay to help his family rebuild their farm. Jiho knows the Kidahara, and he knows how to defend himself against the monsters within the Kidahara. But when he and a team of other kids have to get away from monsters, they get lost. Then they meet a girl who lives in the forest. It’s Princess Koko! Koko leads them to the magical realm so they can save the kingdoms from dark magic. But an ancient evil is rising. Will Jiho and his friends be able to stop it?