By the 1970s when Christie wrote her last three books (Nemesis, Elephants Can Remember, Postern of Fate) it seems extremely likely that she was suffering from Alzheimer's. At the very least, she was obsessively preoccupied with the subject of memory loss. Let's look at the first chapter of Nemesis. 
 Mr Rafiel and his masseur-attendant Jackson ... (12 lines later) ... He had with him a valet attendant, a qualified masseur ... (7 lines later) ... Miss Marple wondered whether Jackson? Johnson? had stayed on ... (10 lines later) ... no, it wasn't Johnson, it had been Jackson, Arthur Jackson. 
 Mr Rafiel and his secretary Mrs, Mrs Walters, yes, Esther Walters ... (a page and a half later) ... now what was the name of that nice secretary that Mr Rafiel had. Oh yes, Esther Walters. 
 Miss, Miss Bishop. No, not Miss Bishop, of course not. Why had she thought of the name Bishop. ... (4 lines later) ... Of course, it was Miss Knight I was thinking of. Not Miss Bishop. Why do I think of her as Miss Bishop. The answer came to her. Chess, of course. 
 OMGWTFBBQ!!1 Who gives a crap?! Can you just get on with the damn story!? Why didn't a relative stop her from trying to publish this drivel? And where were the editors? Why are my eyeballs and little grey cells being subjected to this torture? 
 It doesn't get any better from there. Any time you're reading a Christie novel and learn that a murder occurred X years ago (a plot device that Christie unfortunately turns to far too often), you know you're in for a god-awful read. This is no exception to that rule. The idea that Mr Rafiel would not provide her any instructions as to what he wanted done ("so as not to prejudice her opnions") is stretched beyond the breakingpoint. Particularly annoying is that every few pages, she stops to give us a summary of events to that point. It's almost as if you were reading a serial ("when last we left our heroine blah blah blah"). I guess that's another symptom of Christie's failing memory, assuming her readers need a reminder of what happened in the last chapter. And finally, the one unforgivable offense, there's really only one serious suspect, despite some lame attempts at red herrings. Only to cap it off with explanation ad nauseam of the chain of events, which was obvious to anybody chapters before it was revealed. 
 Aside from the plodding uninteresting plot, the treatment of rape (it's no big deal, and girls usually lie about it anyway) is appalling. And her opinions on lesbianism, while they might have been ok in 1920 when she started her career, were bizarre in the extreme for 1970 when this novel was written (and the reference to "Whatever Happened to Baby Jane" confirms it's set no earlier than 1960). 
 Bottom line: It's better than Postern of Fate, but just barely. Only for hard-core Christie fans. All others can skip it.