A ten-year-old mystery hidden inside a library book. With the school sports festival just one week away, third-year student Kanon—sidelined by injury—steps in as a substitute library monitor. Processing returned books, she finds a copy of Kästner’s Flying Classroom on the shelf that had supposedly never come back: checked out a decade ago by a student who died in an accident while still enrolled, it has reappeared without explanation, a cryptic note tucked inside. Together with her bookish co-monitor Sakutaro, Kanon begins untangling the mystery—and in doing so, confronts questions about tradition, belonging, and the messages we leave behind for those who come after us. A luminous YA novel about the library as a place that holds people together, generation to generation.