Sorcery and Cecelia: the Enchanted Chocolate Pot Regency Era epistolary novel in a world where magic works. Wonderfully funny. There are two sequels, as well. The second book as as good as the first. The third isn't about Kate and Cecy, so I don't like it as much, but it's set in the same world, so it's a fun read.

Have fun with the treehouse. 8-)