One of my favourites is Sorcery and Cecelia; or, The Enchanted Chocolate Pot by Caroline Stevermer and Patricia Wrede. It's an alternative history novel that was great fun.
In 1817 in England, two young cousins, Cecilia living in the country and Kate in London, write letters to keep each other informed of their exploits, which take a sinister turn when they find themselves confronted by evil wizards.
