Aaah, yes, here we go: Postern of Fate
Postern of Fate was the last book Agatha Christie wrote and the initial notes are clearly dated November 1972, almost exactly a year before publication. It is arguable that her agent and publisher should never have asked for another book after Elephants Can Remember. Although H.R.F. Keating reviewed Postern of Fate charitably in The Times with the ambiguous phrase ‘She stills skims like a bird’, there is no doubt that it is the weakest book Christie ever wrote. The most interesting passages are those where we get a glimpse of the private Agatha Christie.
And then Curran basically tears the book to shreds, pointing out all the things that didn't work.
Interestingly, there are hardly any notes about the book in the Notebooks.