Answer:
Plot/Characterization/Symbolism
Step-by-step explanation:
Mainly depends on how you look at it.
A character's description can be crucial to a mystery book, they might act super optimistic and be likable in general but be the killer.
The plot is literally the whole story so that's your best bet.
Symbolism can also be a great deal to mystery and horror books. Similarly to movies, books incorporate a lot of symbolism that represent something, and most of the time, they either foreshadow or show the theme in some kind of way.