The result is that half of the book consists of letters of Charlotte, this not slow down even when bad storytelling. I always struggle to appreciate the flashback, the it is more or less the same principle.
The problem I have is that this book is for young tweens (see children) and that there is still a lot of moral lessons on religion and the vision of this period of history is quite disturbing. I'm not saying it's wrong, I was not born (hehe) but I am not persuaded that this is a book that can let a child read without accompaniment (or pre-reading to meet his questions). Not being a believer I can not accede to the message of the book but I can understand that it may appeal to some.
After the book is very well written and we want to know the future. I recommend just to adults to read before offering it to a child.