I'll be elaborating in my review , but here are some reasons:

The books shouldn't work as well as it does. Croning changes stiles and narrative approaches throughout, but what he maintains is that most important or elements - I didn't want to close the book during my lunch breaks, or at night.

The characters plight was believable, the sense of tension and terror were maintained equally throughout the majority of the novel. He made you want to root for the protagonists. Cronin also managed to hold some of his cards to his vest, he didn't reveal everything and in a less skilled writer, this would have been annoying rather than given me the drive to keep reading.