Alright! I get to air my grievance with Christie's novels!
When I was in high school, I read everything I could get by her, pretty well everything she published. Out of all those books (well over fifty), I guessed the right person _three_ times. So, I eventually figured out that her task was to keep you from not figuring out who the murderer was. I was especially incensed when, at the final group meeting, Poirot would whip out a cable from an ocean liner crossing the Atlantic, and the cable would have some vital piece of information required to solve the mystery (like person A was really the nephew of the deceased and stood to inherit the fortune). Very frustrating for a kid who thought his little grey cells weren't too shabby either. I guess I wasn't smart enough to understand that they weren't puzzles like the mystery books for children I had read before that.