Endless Night did not strike me as your typical Agatha Christie book. It was creepier. It felt more Gothic in the beginning, and then it seemed very modern (for the time). The tone of the book shifted dramatically, suddenly, and that shift made the creepiness resolve into a sense of horror.
The denouement was a surprise to me, Christie's reveals almost always catch me by surprise, but this denouement also seemed a little clunky. I can't even say why it felt clunky, because doing even just that will give away the ending, and I don't want to spoil the book.
This is just a different Agatha Christie. Worth the read just to see how well this woman could manipulate her readers' emotions.