The Girl of Ink and Stars by Kiran Millwood

When her best friend Lupe, disappears into the Forbidden Territories, Isabella disguises herself as a boy to join the Governor’s expedition to find her. Armed with her father's maps and her knowledge of the stars, Isabella navigates treacherous terrain and uncovers ancient myths about a fire demon named Yote who threatens the island's existence. As she ventures deeper into the island's heart, Isabella discovers that the legends are more than just stories—they hold the key to saving Joya.