This is one thing, as I have never really read the complete version of "The Three Musketeers" and I really feel curious about it. I think it must be great as I also enjoy "The Count of Monte Cristo" a lot. Another thing that impresses me is how much research the author has done on studies of the devil, the demons, and all the related subjects. It's really interesting to see the Devil from a different light. (Or it is really not so different...)
It is also interesting to read a story that jumps between the first-person and the third-person narration. It's not so awkward, but again not so logical. If there is a first-person narrator, this person shouldn't be in the least omniscient, even partially. But that's the way the novel is written!
One thing that I don't like is the spoiler provided by the first-person narrator. I don't know why this "I" has to tell the readers from the beginning that Corso isn't going to die. That really lessens the suspense feeling!