This is a book I read for the first time as a teenager, but which was so good I spent years looking for it in used bookstores (it was out of print for a time).
Kate and Alicia Sutton are handmaids to the princess Elizabeth of England. When the beautiful and romantic Alicia sends a letter to Queen Mary about the unpleasant conditions of Elizabeth's house the queen blames the plain, serious Kate and has her banished to a remote castle in the north. There Kate meets a young man who's devised a harsh penance for himself for a crime he committed in ignorance. In attempting to unravel the mysteries of the Perilous Gard, the castle in which she now lives, she comes face to face with the queen of the fairies.
The book comes up with a surprisingly realistic explanation of who and what fairies are and why Christianity affected them so strongly. Christopher's acerbic comments in regards to Kate's meddling bring his character to life and make for some interesting scenes.
Pros: the clever bridging of history and mythology, a very subtle love story, poignant ending
Cons: When I first read the story I found the opening chapters boring. This hasn't been a problem since, but other young readers may be put off by them as well. Persevere, the book is definitely worth finishing.