I have seen the TV versions of the novel several times and in fact did wonder whether it was worth my while reading the book.
I hadn't realised how much the story had been modified for television, with characters left out, and others inserted. There are a number of plot changes.
The main import of the novel is that nothing at Bertram's Hotel in 1955 is as its seems: all is a facade, from the appearance of the hotel, to the people who visit it, to the people who run it. Miss Marple realises that it is a mistake to try to step back to pre-war days. In fact the Bertram's Hotel she remembers is much older than that, a memory from her childhood.
The story also illustrates Agatha Christie's conviction of the prevalence of organised crime rings that underpinned facades of normality. The police inspector who carries out the investigation into Bertram's shady dealings and the disappearance of Canon Pennyfather is an avuncular old chap who has seen it all, but he is not the same as the bouncing lad of the television production. Nor is there the romantic element that TV gave us for public consumption.
I don't think Miss Marple comes out of thebook particularly well - Christie portrays her as an old busybody who eavesdrops on people's conversations when she can. On the other hand she does recognise evil when she sees it and she demonstrates an understanding of the foibles of the elderly. For example she knows that Canon Pennyfather had mistaken the day he should be flying to Lucerne, and when he returns to Bertram's Hotel, she instantly knows he is not the person she saw descending the stairs at 3 am.
So an interesting read. Perhaps not Christie's best.
“Time is the best killer.”