Where's Polly?

I am currently reading the book, "The Magician's Newphew" by C.S. Lewis. When I looked at the top of the book it said, "The Chronicles of Narnia".  I have always watched the movie Narnia so I asumed the book was going to be something similar to the movie. I was completley wrong. So far, the book is about two friends. A boy named Digory and a girl named Polly. They find a secret passageway that allows them to go into different houses. To me, that would be very frightening. Just to sit on the couch watching your favorite show, look to the right, and see two kids going down the stairs of your house. The first two questions that would pop into my mind are who are you and how did you get here?
Polly and Digory decide to take advantage of the passageway and that's exactly what they do. They wanted to go and see who-if anyone- still lived in an old house that was a couple blocks down from Polly's house. They both believe it was haunted, but neither one of them started that conversation. When they thought they had arrived, they saw a door in the passageway that was suppose to let them enter the house. When they opened the door, they saw Digory's Uncle and realized that they were in the wrong house. Digory thought his uncle would be very cross, but he actually seemed very friendly. There was a table in the room, near Digory's Uncle that held two beautiful rings. Polly was so curious about the rings and Digory's uncle knew. Digory's uncle told Polly she could have one of them and her face just lit up. Polly touched the ring and suddenly vanished. Digory was so scared and he felt as if he was going to be next. He asked his Uncle Andrew where Polly went and he said he didn't know. Uncle Andrew began to tell the story of the ring. He said that a woman named Mrs. Lefay told him to get rid of the rings before she died by burning them, but he didn't listen. Uncle Andrew also said he had tried his before, but not with a human being. Where did Polly go?