A machine of a plot, and the machine is flawless; the people in it are cardboard and I did not mind

Well, it works. It has worked on every single person I have handed it to, including a boy called Aaron who told me to my face in September that he did not read books, and who finished this in two days and came back wanting another, and that, more or less, is what stock selection is. Takes twenty years to learn.

Nobody comes to Christie for prose and it's a bit silly to complain that there isn't any. The characters are types, the judge and the doctor and the governess and the mercenary, dealt out like a hand of cards. What she has instead is architecture. Ten people, one island, a rhyme, and a structure so tight the whole thing could be diagrammed on a single side of paper. I've never read a better-engineered novel and I've read plenty of better ones.

The middle is where it genuinely gets good, and it isn't the killings. It's the paranoia. Seven left, each aware that one of the seven is doing it, sitting in a room working out whether to go to sleep. She is very good indeed on the temperature of a room. Vera on the beach, the sewing, the bees.

Where it fails, and it does fail, is the epilogue. The confession in a bottle is a cheat and a dull one, several pages of a man explaining a conjuring trick after the audience has gone home. The solution also requires one character to behave with a stupidity that appears nowhere else in their portrayal. I shan't say who. I worked it out on a second reading and was quite cross about it.

Ruth listened to it on a long drive and rang me from a service station to announce that she had worked it out at the halfway mark. She had not. She had worked out something else entirely, and I let her keep it, because there is no percentage in winning that particular argument with your own daughter over the telephone.

The title history is worth knowing and I am not going to set it out here. Our copy came in under the second title, which we withdrew in about 1991 after a complaint that was entirely correct, and the replacement sat in that same slot for the remainder of my time. Mr Sowerby made a fuss about censorship, then read the original title out in the staffroom, and stopped making a fuss.

On the shelf it is a workhorse. Cheap, short, always in print, and it manages the thing the reluctant-reader industry keeps trying to buy with expensive schemes: it makes the next page more attractive than the alternative. I kept six copies. In a good year I lost two.

Eight. If you want to understand how a plot is built, take this one apart; I'd not go to it for anything else, and she would not have expected me to.