Based on 1st read
I came into this novel thinking that it would be horror and hearing it compared to Interview with the Vampire. There are supernatural elements. It isn't Stephen King type horror. I think the premise and pacing was very similar to Interview with the Vampire (which I also struggled to read); It was a well-done book, but it wasn't really for me. While reading, I got bored with it and took frequent breaks. I think the pacing felt slow because I had an idea for where the story was going. I was intrigued by the order of immortals and their schools and origins. My interest was really piqued by the ending though, and I think I would continue with this series.

Based on 2nd read
Before I was of the opinion that you could just read a Wiki summary of the 1st book and skip to the 2nd, and I don't know what shifted between my 1st read and this read because my opinion drastically changed. Don't skip this book if you're going to read the series. The main horror elements of the book are the emotionally abusive relationship Jessica is in, the psyche of her abuser, and the amount of danger she endures to maintain a facade of normalcy.