This post is probably going to be spoilerific. Read at your own peril.
Death on the Nile is one of my favorite Agatha's, which is a bit unlikely, actually, since it is solid, but not even arguably among her strongest, mysteries. I've read it several times, and always enjoy the various characters. I also love the setting - a closed universe of individuals on a barge, the Karnak, travelling slowly down the Nile River in Egypt.
Having read Endless Night earlier this year, I cannot help but be struck by the similarities between that book and this mystery. In both of them, the characters execute a long and complex con to obtain money belonging to a wealthy heiress. In Endless Night, of course, the main character is an unreliable narrator, a character study of a sociopath who is the architect of the con. In Death on the Nile, told in third person, Hercule Poirot puts together the pieces to solve the mystery. There is one particular clue that puts him on the trail - both the male and the female characters use the same analogy to describe the manner in which the heiress - the lovely Linnet Ridgeway - has stolen Jacqueline de Bellefort's fiance, Simon Doyle.
This book also includes the redoubtable Colonel Race, who is one of my favorite of Christie's recurring characters. He appeared, first, in The Man in the Brown Suit, a book I have not read. His second outing was in Cards on the Table, along with Ariadne Oliver & Inspector Battle (another of my favorites), which is the only book where the four of them appear together. This is the third appearance of the good Colonel, and he shows up for the last time in Sparkling Cyanide. This secondary mystery involves a foreign spy who is also travelling on the Karnak. There is even a tertiary mystery involving a jewel thief that ends up being solved by Hercule Poirot during the trip, and a spot of financial impropriety to layer on additional motive for murder. There are clues and red herrings galore.
There are a few themes that repeatedly appear in Christie's books that show up again here, including the apparent overwhelming problem with blackmail that existed among the 1930's British population. It seems that the first impulse of anyone who learns of criminal behavior by someone else is never to call the police. Rather, they try out a little extortion. This almost invariably ends badly for the blackmailer.
Murder for gain is also the most common motive, and the person who has lots of money might just as well paint a target on his/her back because the heirs are constantly attempting - and in Christie land, succeeding - to bump them off, especially when they aren't interesting in passing some of that money down before they die. One wonders how Queen Victoria managed to last for 80 some years given this British propensity to hasten the death of the person in possession of the family fortunes.
Anyway, there is a lot going on in this book. It's a lot of fun,