Dead Man's Folly is a Hercule Poirot book by Agatha Christie, but perhaps it should have been called "Christie's Folly"... While the premise is fantastic and the idea behind the motive is clever, unfortunately half the cast is pointless, the book never follows through on the premise, and the solution is severely underclued.
There are spoilers tagged with rot-13.
The book begins with Poirot receiving a call from Ariadne Oliver, a murder mystery writer who is Christie's self-insert, to come down to a country estate, the Nasse house, where she is helping to plan a murder mystery game for the owner's party. That should be all you need to know about the plot. Oliver feels 'something' is wrong, but can't put her finger on it. Lo and behold, the local village girl who was playing the victim is found murdered in the same location and manner as the victim in Oliver's script.
(I want to take a moment to clarify the fake murder mystery. This was not the traditional sort of murder mystery party with a small number of guests who are each given a fake role/character to play. Rather, the owner of the estate was holding a party like a carnival, and the murder mystery game was like a murder-themed treasure hunt, where the clues lead to the evidence, and the goal was to gather the evidence and then solve the crime.)
This is a phenomenal premise. We have a fake murder turned real. The reason Oliver called Poirot down was because she felt someone was trying to influence her murder mystery plot for some nefarious reasons, but she couldn't figure out who or why. We're left with a problem of exactly how the murderer got Oliver to change her murder plot and how those changes tied into the real murder.
Unfortunately, this premise never goes anywhere. We're never given the specifics on what Oliver changed. We're never given specifics on why those changes were necessary. There's never any sort of issue on whether a certain piece of evidence is from the real murder or the fake one. And the clues that do tie the fake murder to the real one end up being ridiculous clues towards minor points that make absolutely no sense. [Spoiler] Sbe vafgnapr, gur pbzvp obbx jvgu "ybbx va gur ehpxfnpx" jevggra va vg. Ubj pbhyq 'Unggvr' CBFFVOYL svaq gung vapevzvangvat? Pbafvqrevat vg jnf yrtvgvzngryl n pyhr va gur snxr zheqre, ubj jbhyq nalbar guvax gung pyhr jbhyq zrna gb purpx gur ehpxfnpx va gur furq gung abobql xarj rkvfgrq be gur bar orvat jbea ol bar bs gur enaqbz sbervta tveyf, rfcrpvnyyl fvapr fur jbhyq unir orra ybat tbar ol gur gvzr guvf jbhyq or eryrinag?
While about half the suspect list is fine and sufficiently developed, the other half seems completely extraneous. We start with Poirot and Oliver, of course. I think Oliver is my favorite Christie character, and every time she shows up is a delight. While I love seeing what Christie has to say about herself and her craft, Oliver alone cannot hold up the story. Nasse house, the setting of the book, is owned by Sir George Stubbs, who has recently purchased it from Amy Folliat, a widow who is staying on the land in a shack. Stubbs is married to Lady Hattie Stubbs, a beautiful simpleton (or is she?), who goes missing around the time of the murder. A Christie twist often involves a personality or relationship actually being the exact opposite of what it appeared to be on the surface, and I think Christie does a fantastic job of casting doubt on Hattie's true personality, even for someone expecting a Christie twist. Finally, we have Amanda Brewis, George's secretary. I would call this the 'core' cast of the story. Their personalities and dynamics are sufficiently developed and fleshed out for the purposes of the story.
I cannot say the same for the rest of the suspects, however. These include Michael Weyman, an architect hired by George, Alec and Peggy Legge, a couple renting a cottage for the summer, Mr. Masterton, the local member of Parliament, and his wife Mrs. Masterton, Captain Jim Warburton, Mr. Masterton's agent, and Etienne de Sousa, Hattie's cousin who has come to visit. These characters get little screentime, one scene at most to demonstrate their personality, and minimal relation to the plot. Some of these characters (like Michael and Etienne) I think actually get enough screentime for their purpose in the story, but overall I feel the story would have been better served by either tightening up the suspect list or further developing the existing ones. As it stands, all most of these characters do is set up a red herring that is obviously nothing more than a red herring. (There are some other side characters, like the victim, but as side characters they're obviously not expected to have a large role or developed personality.)
As for the solution, it falls flat. The idea behind the motive is clever, but nearly every element of the crime is plain, unnecessary, or underclued. For the motive, Christie performs a fantastic sleight-of-hand, keeping you focused on the wrong thing while the solution is conceivably in sight. Unfortunately, the motive itself is not clued at all. Poirot seems to pull it out of thin air, based purely on conjecture. I want to emphasize this, because the motive seems to come out of nowhere, and essentially contradicts some of the basic facts we had come to known without any warning.The physical execution of the crime then just consisted of the killer going and killing the victim, along with some other parts that are the poor combination of entirely unnecessary and unfairly clued. [Spoiler] Gurer jnf ab ernfba Unggvr unq gb frg hc gur crefban bs gur Vgnyvna fghqrag. Juvyr qvfthvfvat urefrys nf na Vgnyvna fghqrag jnf svar gb farnx bhg bs gur cnegl jnf svar, gur cnegl jnf n ynetr, choyvp nssnve jvgu frireny uhaqerq crbcyr, naq sbervta lbhguf jrer pbafgnagyl tbvat gb naq sebz gur ubfgry. Rira vs fur unqa'g 'rfgnoyvfurq' ure nygreangr crefban orsberunaq, abobql jbhyq abgvpr vs gurer jnf fhqqrayl n arj Vgnyvna fghqrag ng gur cnegl. Nqqvgvbanyyl, gur pyhvat gung Unggvr jnf gur Vgnyvna fghqrag jnf hasnve. Va gur bcravat bs gur obbx, gur Vgnyvna fghqrag'f unve vf qrfpevorq nf purfgahg-pbyberq naq pheyl, juvyr Unggvr'f vf oynpx naq fgenvtug. Cbvebg znxrf n cbvag gung vg jbhyq unir orra rnfl gb jrne bar bs Unggvr'f qerffrf bire gur fghqrag'f fvzcyr pybgurf naq znxr-hc, ohg V qba'g guvax gurer jnf ebbz sbe n jvt.
If this wasn't already enough problems, it also feels like Christie didn't finish polishing out all the details in the book. Sometimes the characters just start mentioning or talking about details as if they had already been established, even though they had never been discussed before. For example, one point of contention becomes Miss Brewis's testimony that Hattie asked her to bring snacks to the girl who became a victim shortly before the murder. Miss Brewis never says this (maybe I missed it, but I read that part of the book four times and didn't see it), but it gets mentioned by other characters several times. It's especially important because Hattie goes missing, and (as far as I could tell) she would have told Miss Brewis this around the time she went missing, so Miss Brewis's testimony on this point could be an important clue in Hattie's disappearance. There are some other details like this, but overall it seems like Christie did not iron out all the details in her telling of the story.
I had high hopes for Dead Man's Folly, since it opened with my favorite Christie character and a compelling premise, but in the end it just did not measure up. Between weak characterization, a poorly clued solution, and a general lack of polish, I recommend skipping the book, but reading the Wikipedia summary to see the motive, which was at least a clever idea.