Which of her novels did Agatha Christie describe as the most difficult to write?

"...Ten people had to die without it becoming ridiculous or the murderer being obvious. I wrote the book after a tremendous amount of planning, and I was pleased with what I had made of it. It was clear, straightforward, baffling, and yet had a perfectly reasonable explanation; in fact, it had to have an epilogue in order to explain it. It was well received and reviewed, but the person who was really pleased with it was myself, for I knew better than any critic how difficult it had been... I don't say it is the play or book of mine that I like best, or even that I think it is my best, but I do think in some ways that it is a better piece of craftsmanship than anything else I have written."