Prince Caspian: The Original Novel by C.S. Lewis
Polly's hand went out to touch one of the rings. Immediately, without a flash or a noise, she vanished. When horrible Uncle Andrew starts experimenting with magic, Digory and Polly find themselves in another world, and at the beginning of an incredible adventure, as the doorway to the magical land of Narnia opens This is the first adventure in the exciting Chronicles of Narnia.