It is true that she could be formulaic, especially in her early books. Miss Marple’s eyes endlessly twinkled and Hercule Poiret kept twirling that famous mustache. What keeps her books readable, though, is that she never cheats (the clues are always there if you know what to look for). That’s the obvious reason. However, I also believe that, despite her formula and repetition, she understood human nature, something that became increasingly obvious as her writing matured.