First, three and a half stars. There's one main reason it's not four, and that's the plot.
Halfway through The Spiral, I was extremely confused. I think the author aimed to make a point by changing the style of the writing, but what that point was I'm not sure. I don't really understand the need for so many diverging plot points, I just got totally bamboozled.
That being said, the ending was brilliant and pulled together so many things about Erma's private life, and that brings me to the characters.
Erma started of basic, rude and generally unlikeable. She had a really great character arc, and by the end although rude, she was stronger and more mentally developed. Some characters seemed unnecessary, but all of them had emotional depth.
The Spiral began as a classic murder/thriller book, I have no idea what it was by the end. It warped into fantasy/mystery, and was extremely dark and attacked many gritty themes and mental health problems.
Finally, the 'villain'. And what a villain it was, psychotic, cruel and charming, and not who I thought it was going to be.
All in all, a good but not light hearted read full of morally grey characters and intrigue.