it is definitely a very interesting book. According to my experience, it isn't a typical piece, it is hard to mark it as one genre. 
 First of all, it is in the atmosphere of a gothic story. But it isn't "too much gothic" so someone like me (I mean, someone who isn't a fan of the gothic story) isn't bored. I confess I was even sucked a little in this atmosphere ;-) Anya Seton wrote in the way that I felt like I was been with characters, like I was felt fear with them. 
 "In after years Miranda knew that her first sight of Dragonwyck was the most vivid and significant impression of her life. She stared at the fantastic silhouette which loomed dark against the eastern sky, the spires and gables and chimneys dominated in the center by one high tower; and it was as though the good and evil, the happiness and tragedy, which she was to experience under that roof materialized into physical force and struck across the quiet river into her soul" 
 Secondly, it is a great historical fiction. Seton made the past nearer. She described the world in the way that I can almost feel that I saw it on my own eyes. And most of all, I can understand characters not according to my modern views and opinions but according to their views and belief. Like in her Katherine it is the strongest party book. 
 And last but not least, persons in this story are human. They aren't black or white. There isn't a villain like in a some Hollywod movie. Like in the real life, everybody is good and bad. Everybody has some dark sides. But everyone deals with it in different way, because everyone is different (you can call it genes, soul - as you like) and has different life/experiences. So, one person becomes better person and other not. But we all are just human and at the end we all are the same... 
 It is really a good book. I will be remembered it long. Especially, when I will see or read about (view spoiler)[oleander (hide spoiler)] or anti-rent war or steamboat races I will always recall this book.