The fact that it's based on a true story. People are often intrigued by real - life events, especially when they involve a nefarious plot. It gives a sense of authenticity that fiction sometimes lacks.
Well, it's also about the human element. These books often show how ordinary people can be involved in extraordinary and evil plots. We see the good and the bad sides of humanity. In 'The Devil in the White City', we see the contrast between the grandeur of the fair and the horror of Holmes' killings. It makes us question human nature and that's what makes these books so captivating.
It's the mystery and the darkness. In these books, we get to explore the motives and actions of those involved in the nefarious plot. For example, in 'In Cold Blood', we try to understand how two men could commit such a cruel murder. The real - life aspect makes it more engaging as we know it actually happened.