Ruins of Zu Zone, Museum
Inside the museum's base room, there won't be much except a fireplace that goes nowhere, your exit, and a suspicious bookcase. Access it much like you would a conversation with Dot or utilizing levers to reveal a secret door. Inside is a cube planet replica, where you can get a bit from by rotating it left once, and also get into another secret room by rotating it twice. The chest inside the next room will have the Tome artifact.