I enjoyed this book. The mystery was intriguing and while at times I think the action dragged a little for the most part I was highly engaged.
Although the story starts out following a sixteen year-old girl, she eventually fades into the background and the bulk of the narrative is really about her father and Helen. There's a fair amount of correspondence in the book but it all sounds as though it were written by the same person. There's also a bit too much dry history. It also gets a little repetitive to have most people they encounter say "you know, Dracula was based on a real person." I was already aware of that, as, I imagine, are a good number of the readers of this book and even if they weren't, having it happen once or twice in the story would have sufficed.
The story involves a great deal of travel and it's easy to see the author's admiration for Eastern Europe's history, traditions, architecture and peoples. Reading about that history and reading Kostova's portrayal of life under Communism was at least as interesting as the Dracula mystery itself.