It was an interesting experience, re-reading this book now, after Death Bringer, in the light of certain revelations that Death Bringer brought with it.
I enjoyed Mortal Coil the first time I read it (and the second time!), but this time, I think I appreciated it even more - especially the bits of conversation between Valkyrie and Skulduggery, and the things he tells her, and the way he understands what she's going through.
And of course, the very final part of Mortal Coil reads a whole lot differently now - as do Skulduggery's actions. He's a changed man, compared to the first three books; colder and harder (and yet even more understanding and caring and kind when it comes to Valkyrie - especially after he learns of her secret), which is especially obvious now that it has been pointed out in the next book.
I've seen quite a few complaints about the escalated level of violence in this compared to the earlier books in the series, and it's definitely there. There's more gore, and more death, and more shocking brutality, especially once Darquesse comes out to play, and more horror, but... I'm not usually someone who reads books for their violence and gruesomeness, and I hate the idea of using violence to make one's books "gritty" or "edgy", but in here, it works. Every bit of it is there for a reason, and it has consequences, and it's not played for laughs.