Mystery fiction presents an interesting challenge. There are certain conventions or "rules" governing the genre—including a literal set of ten commandments (and another ruleset double that)—and yet, from one point of view, the ultimate goal is to surprise and shock the reader, while remaining within the "rules." Obviously, no one is putting a gun to someone's head to obey these rules (well, maybe in a story they do), and some of mystery fiction's most famous and celebrated works break these conventions. So they're more like guidelines. There's no reason you can't just say a wizard did it, but doing that will result in an unsatisfying resolution. (Usually.)
The Eighth Detective is a short story collection that aims to expand the "rules" of mystery fiction by providing a comprehensive definition of a murder mystery and a series of stories that illustrate this definition. It's pretty underwhelming. I think this is because the central conceit suffers from a critical flaw: the formula that The Eighth Detective is built around tells you whether a piece of fiction is a murder mystery or not, but has absolutely nothing to say about whether it's good.