First, the good. This novel introduces a plethora of excellent characters surrounding a Crescent where a most peculiar murder takes place in the home of a blind woman. The leading investigators are a Detective Inspector and a marine biologist who seems to work for both MI5 and MI6 - perhaps Christie was unaware of the distinction? There are, of course, ancillary murders - which led me to discover half the puzzle. Without giving the game away too much, I will now try to describe what's wrong with this book.
Christie was in her 70s when she wrote this, and obviously sick of Poirot - he doesn't make much contribution until the "action" is almost over. However, the main problem here is that the puzzle is unbelievable, and therefore, logically unsolvable. The culprit has some knowledge that they couldn't possibly have and the reason for the initial killing, and the circumstances surrounding it are, in my view, ridiculous and involve some irrational behaviour that is totally unexplained. There is also at least one error in the run-up to Poirot's summing-up - but this is sloppy, not fundamental - and, as I said, Christie was in her 70s.
Almost to the presentation of the solution, this is an excellent novel - well-written, good characters, and an interesting plot - let down in the home straight. The feeling I get is that Christie lost interest when it came to explaining the tangled web she wove! One almost gets the feeling that, as she aged, she could have developed a second career as a writer of excellent "straight" novels - she certainly had the insight and the technical ability. As I said in the title, mostly worth reading.