What an unique book! Mixing genres is always tricky but if one does it well a result can be marvelous. And here we have one of the examples. 
 Imagine: 
 * a sweet Regency romance 
 * two witty and inquisitive young girls, best friends 
 * the world of manners (rules for girls, behaviors and so on) 
 * a subtle magic which exist but doesn't rule the world 
 * and some mystery to solve (which brings danger and shows you who you really are). 
 What you can get from this mixing? I can easy imagine many disasters, failures. But it isn't in this case. Maybe because of the way it was created, in a particular way. Patricia C. Wrede and Caroline Stevermer made some kind of experiment or fun: the Letter Game (in short, they wrote to each other as invented characters). As a result one of those games they created this novel. 
 I simply love this book. Lighthearted, witty, engaging, lovely, charming, funny. 
 "My first thought was "Earthquake!" My second, "I will never be able to wear this bonnet again" ;-) 
 I am not a fan of fantasy but I can appreciate when magic in the book gives it something precious. Magic here is (in my opinion) an instrument to show us something, to make a book a little more interesting too. But it isn't overwhelming. It is just like it should be. 
 [The same beautiful way of using magic I found also in: [book:Chime|8299165],Chime, Shades of Milk and Honey, A Curse Dark as Gold, Ella Enchanted, The Probable Future, The Picture of Dorian Gray, Green Angel. They are different books, some sad, some with happy ending, but all of them are really good]. 
 I would like very much to read more such novels. But I am afraid, they are very rare.