In the Woods has all the hallmarks of a mystery novel—a murder, a disappearance, an assortment of oddball suspects, police procedural straight out of cable television, and detectives whose personal lives are just as screwed up as the cases they're investigating. It also won an Edgar Award, which is awarded to the best works in the mystery field. So that pretty much settles that.
But hold up, because it's not your average mystery for one big reason: The case goes unsolved. And that goes against the tenets of the mystery genre.
Like Dudley Do-Right, mystery detectives always get their man (or seventeen-year-old girl). But in In the Woods, not only does the mastermind behind the crime, Rosalind Devlin, basically go free because Rob screws up the investigation so much, but the 1984 crime, an incident that hangs over the entire plot of the novel, goes completely unresolved. It basically could have been anyone—Rob, Jonathan Devlin, an owl-shaped demon in the woods. Even a true detective like Rust Cohle couldn't have solved this mystery. It's decades old and cold as a case gets, and yet it looms large.
Because of this, In the Woods is a postmodern mystery—it takes the traditional tenets of the mystery genre and toys with them. After all, not only does it explore the unexplorable in its repeated visits to the 1984 case, it's decidedly comfortable in leaving the whole debacle unexplained.