Princess of Souls is overall and enjoyable read with an interesting premise and characters, and while the plot and characters fall a bit flat, I liked the story enough to round 3.5 up to a 4 on Goodreads.
I got this book after reading a preview of the first several chapters, and I was hooked on the plot and characters. I also enjoyed Christo's writing style; the narrative flows smoothly and is well-written, allowing for quick and easy consumption of the story.
As for the story itself, I found the first half to be more compelling than the second, both for plot and character development. By the climax, certain tropes are incorporated into the story that are personal pet-peeves of mine, so they may satisfy other readers a bit more. I don't want to add anything spoilery, so I'll just say that I ultimately wished for slightly stronger/unique character motivations, actions and resolutions.
Personally, I found Princess of Souls to be a good way to spend an afternoon, and as my first book by Alexandra Christo, it was entertaining enough to prompt me to look up more of the author's work.