"The Tombs of Atuan" is book two in the Earthsea Cycle by Ursula R. Le Guinn. In this book we meet a girl named Tenar who has been chosen to be high priestess to the Nameless One's. It's a solemn duty, and Tenar grows up in a pretty restrictive atmosphere. Then, once she's old enough, she assumes her position and all the duties that come with it. One of Tenar's duties is to learn the underground passages in the Sanctuary of the Nameless One's. The other priestesses who guide her there originally are only allowed to go to certain portions, on pain of death, so much of it she has to learn by discovery and by instructions passed down by mouth. Oh yes, and did I mention that no light is allowed underground? Tenar slowly learns her way around all the normal passages, and through the great Labyrinth where treasure is stored and kept sacred. Then, one day, she discovers that a sorcerer has managed to penetrate the underground, and is searching for the treasure. She's conflicted as she tries to figure out how to deal with him. She starts by trapping him inside the Labyrinth to give herself time, and then decides to keep him there as her prisoner. Eventually, as she gets to know the sorcerer, she begins to realize that all the things she's believed to be true can be seen in another light. This book is a beautiful story of self-discovery that I think you'll really enjoy!