Well, what can I say? Dead Man's Folly is a fairly typical Hercule Poirot novel. A cast of suspicious characters who aren't being completely open with the police, who are of course baffled by the mystery of who killed the victim. And then the little grey cells do their usual miracle and there you go. Another successful episode in the life of Poirot.
Sometimes I just need a little bit of fluff like this, and as such it fit the bill admirably.