I understand there's a movie coming out about this book, the timing of my reading it is completely coincidental. I actually pulled it off my shelf to read it Sept 1, but didn't start in earnest until the 22nd. I hope the movie is as conscientious about historical accuracy as the book tries to be and isn't a Hollywood horror movie.
The book is extremely readable for non-fiction based almost exclusively on 19th century documents. He has done amazing amounts of research and the notes in the end were almost more interesting to me than the content of the book itself. There is also an extensive Bibliography and an Index.
This is the form of historical non-fiction I would aspire to write