Astonishingly good characterisation
The bar must have been set high in 2016 since this superb novel, which made the Man Booker shortlist, was not the eventual winner. Having said this, if you are after action, this is not the book for you (especially since what action there is is related at a distance of 50 years), but if you are interested in reading an in-depth character study, which truly admits you into the inner workings of Eileen's mind, it is horribly fascinating and slightly voyeuristic to share her memories of how her life unraveled. She is not a pleasant character by any means, but certainly enigmatic. I doubt that many people meeting her would have any idea at all what was going on in her head. The sense of tension and foreboding increases as she gives her first person account and the reader's expectation that at any moment the nature of the crime will be revealed adds to the atmosphere. When it arrives, the crime scene itself - and the wait to discover what impact the event has on Eileen's life - maintain the tension.