Wow. Just wow. Reading this was like a master class in what a psychological thriller should be. None of the theatrics or flash of modern thrillers. Just pure mind-f*#king fear dripped in increasing volume as the novel progresses. 

It deviates from Christie's usual oevre, which I would class as decidedly cosy murder mystery. There is still the big, isolated house and the closed group of suspects, but right from the start, every single person apart from the first victim, are suspects right up to the very end. 

I guessed the rough solution about three quarters in as it was the only viable solution. However, I didn't know who the culprit was until the second last chapter, before the reveal. As such, I was kept at the edge of my seat to the end of my one-day reading of this perfect novel. 

I just hope I forget enough of it in a few years so I can read it all over again and be amazed again.