A murder mystery? Or a murder mystery series? What you recon, HBO? Netflix? Well its something different. And a good murder mystery never fails to fascinate people.
But I don't buy it that Christie didn't know who the murder is until she wrote the last chapter. If she said so... it was probably to put off people who want to copy her style. Her stories clearly (in hindsight) point at the murderer all along, you just can't see it at first. Which is probably the special touch in her stories.
And the "grand conclusion", when all suspects gather in a posh room and the detective reveals what actually happened. Which always made me wonder why the real murderer would go there, knowing he/she will be exposed... :)