The Enchanted Castle
by Edith Nesbit
Three children explore an old country mansion during the school holidays. When they discover her, they encounter a mysterious young girl who claims to be a fairy princess. He shows them the treasure of the castle, including a ring that will make you invisible. But no one is more surprised when he rubs his finger against it... and disappears! The ring is really cute and kids don't waste time figuring out what else they can do.