E. Nesbit 
Enchanted Castle 

Three children discover an old country estate during their school holidays. When they’re exploring it they come upon a mysterious young girl claiming to be a fairy princess. She shows them the castle’s treasure, including a ring that will turn you invisible. But no one is more surprised than she when she slips it on her finger … and disappears! The ring is indeed magical, and the children waste no time finding out what else it can do.