Sorcery and Cecelia, or The Enchanted Chocolate Pot
by Patricia Wrede and Caroline Stevermer
A friend recommended this book to me, and I'm so glad she did!
The novel began as a writing game. The two authors wrote letters back and forth to each other, one as each character, without discussing plot or anything. This is the result. It's the story of Cecelia and Kate, two cousins living in Jane Austen's England -- but with a magical twist.