Lord Zuko moves to catch the book as it drops down towards them, and he manages to catch it in his arms. He's surprised his idea even worked, considering his relative inexperience with spirits. But some of them are quite easy to read, and have a childlike curiosity when it comes to anything human. Apparently this spirit is one of them.

"Thank you for returning this," he says to the spirit. Even if it was stolen to begin with, spirits operate on their own logic, so there isn't much point reprimanding it. Then he turns to Blake, handing the book to her. "Here. It isn't damaged."