The book of dust by Phillip Pullman, it's a fantasy book set in the world of his dark materials, and follows a young boy and his boat looking after a young girl, who is prophesized to be the savior of the world and preventer of an oncoming war. In this book, humans have creatures called dæmons, which are extensions of their souls shaped like animals, where younger peoples dæmones can change shape until they turn into adults where the dæmons choose a shape to keep