The Three Coffins by John Dickson Carr
The Three Coffins is considered one of the greatest "locked room" mysteries. It's less "who done it" than "how it was done." I'm more interested in crime novels generally, but I'll be honest there was a certain thrill of figuring parts of the crime out. Also, there is a great section of this novel that breaks the fourth wall as Dr.Fell, the criminal detective genius, explains that all of the participants are characters in a locked room mystery and goes on to explain the tropes of the genre.
I'm glad I read it, but I don't know that this kind of mystery is my thing.