For those who have not yet had the pleasure, _Sorcery and Cecilia_ is book one, an epistolary Regency historical/fantasy romance. Long out of print, I received my copy years ago by mail from someone I knew solely through internet newsgroups who thought of me when she spotted a copy at a used bookstore (having a copy of her own already). I am overjoyed to say it has been in print again now for a few years, with two sequels, _The Grand Tour_ (Kate and Cecy's wedding journey), and now _The Mislaid Magician_.
Part of the appeal of these books is that they just don't bother to develop the details of stuff like the magic system -- and they don't need to. The world is a familiar one (Regency England). The story is hideously complex and opaque because the books are the result of playing The Letter Game (so the authors haven't necessarily worked out the details of the plot together ahead of time). Nevertheless, the story is involving and entertaining, in part because the viewpoint characters write such deliciously fun letters.
I particularly liked that the twins got all involved in scrying, and that was used so extensively: they played hide-and-seek, they used it to find people, it was used to transmit vital information (not entirely successfully). Etc.