This particular genre seems to attract writing partnerships - perhaps for one to write and the other to research. Whatever the reason, these two authoresses' - both known in their own right in this genre and others - should be highly commended for their ground-breaking novel in this subgenre. Alas and alack, this book is, like the Ordinary Princess woefully overpriced. A link to Bibliofind has been included, below, but you might do well to periodically check Amazon's or E-Bay's auctions, as well as the smaller but more sophisicated Advanced Book Exchange.
Sorcery and Cecelia
Literary Quality: 
Christian Morality: Harmless
Age Appropriateness: Teenager
Modified from E. Snyder's Amazon review.
"Sorcery and Cecelia," an epistolary Fantasy Regency Romance (*phew!*), has quickly joined the ranks of my "novels to read when you want to stay up all night and languish all morning." Quite simply, the book is charming. Rarely do collaborative efforts seem to work, but "Sorcery and Cecelia" most assuredly does! Following the adventures of two cousins - the inept Kate in her first season in London, and the headstrong Cecelia as she discovers her sorcerous abilities - the reader is swept into a world of danger, mystery and intrigue (and the never fully explained story of WHAT in the world that adventure with the goat was about) that makes the pages turn of their own accord! The historical research is solid, the content wholesome and exciting, and the romances (Kate's especially) to sigh for. In short, as the others have said before me, this is *fun*.
Unfortunately, good novels seems to be out of vogue today, and you'll have an either difficult or exorbitant time buying it. For those who have read Wrede's other two Fantasy Regencies, "Mairelon the Magician" and "Magician's Ward," "Sorcery and Cecelia" is a must-read. For those with slim purses, interlibrary loan is a beautiful thing.
Again, however, Cecelia does dabble in spells (making charm-bags to break hexes, if I recall), which may cause some discomfort for the discerning reader. Please see the above links for more information on the Christian view of this plot-dilemma.