I loved the format of this book: The two authors began this book as a game, each writing the letters of of one of the fictional cousins, without ever discussing the plot. The plot itself is somewhat predictable in places, but the characters are so enjoyable that it doesn't really matter. It could almost have been a sequel to "Jonathan Strange and Mr Norrell", going by the setting and the way magic worked. I am very eager to read the other books in the series. 
 Recommended for ages 12+