There's a sadness for things past that pervades this book, mostly through the inner monologue of Miss Marple, but I don't think that the sadness is Marple's, I think it's the author's. She's not really at home in the 60s with it's Beatles and racing cars and therefore nor is her viewpoint character; Miss Marple belongs to the period immediately post WWII, as Poirot belongs to the thirties, and both Christie and Marple know it. 
 As usual, Christie paints glorious characters - Bess and Elvira are among my favourites in her entire canon - and the plot takes in various things that would have been current when the book came out - great train robberies and crime syndicates - but I think the best thing about this tale is the way the crime story is merely dressing for Miss Marple's journey from visceral nostalgia to acceptance that one can never go back.