Comedy-horror, usually styling more towards penis and poop jokes with monsters, but developing some quality Nightmare Fuel when it pulls things together towards the end.
John and Dave can see things like ghosts and travelers from other dimensions. This makes them targets for extra-dimensional invaders and the go-to guys for whatever weirdness happens in your life. See how they got embroiled in all this insanity when the apocalypse road trips to Vegas, and how it comes looking for them at home afterward.
The book is mostly comedy. Dave is a self-deprecating deadpan snarker. John is clearly insane in a mad world, which sometimes makes him genre savvy. I may keep tossing those TV Tropes links because this is one of those books that points out genre traditions as it uses and subverts them. Dave occasionally comments that reality is just as retarded as John always said it was. So the setting itself is surreal, John tries to be stranger, and Dave tries to insert sanity.
The comedy also involves gross-out and shock, what with the penises and poop. John and Dave write Cracked.com, so this may not shock you. Some things and people explode. A dog's butt gets into the action a surprising number of times. When that will not do, insert insects. Sometimes that is more literal, and insects get inserted into people or tear their way out. You know, standard horror stuff with gore.
Most of it is relatively light-hearted, despite exploding people. It takes a long while for the story to take itself seriously enough to get into serious existential horror. It does not dwell upon that.
It is best not to dwell on it because consistency is not a strong point. Dave describes himself as an unreliable narrator at several points, because he is not above lying to simplify the story. The settings messes with demons, ghosts, and aliens without any apparent need to keep them separate or straight, and the explanations often raise more questions. Like why enemies that effective are not being more effective, or why the past can change but random memories remain. The soy sauce that lets them see other dimensions (that's right) is its own version of this, letting them predict the future or work with perfect knowledge, but completely at random as things still surprise them. The enemy has its own version of this.
Some of this comes from the book's development. It was an online series that was later edited into its present format. I am led to believe that people who followed the development will get much more out of this, although they will have already seen more of it. This is a finished product that adds a coherent structure to what was rambling and free-wheeling. Or so I am told; it joined my reading list sometime between the original printing and when it was picked up by a major publisher.
Much of it is silly fun. At one point, John is attacking monsters with a steel folding chair while delivering action movie one-liners. After they safely get away, he thinks of a few more puns, and he lets more monsters in so he can use them.
The romantic plot is shaky, mostly because it arrives in a single day, movie-style. Quite a bit of the book is in movie style, and the director of Bubba Ho-tep and Phantasm got the movie rights before this edition was published, so it might be worth just waiting for the movie.
The horror elements are generally not scary, unless a description of a monster scares you. Some things will be sufficiently creepy, notably when masses of bugs or meat inhabit the uncanny valley. Some things will lose their coherence if you stop to think about them, which the book conveniently avoids (and keeps moving so you won't). As I said, the later parts get to some existential horror that could be downright disturbing if you pondered them enough. The book rarely dwells on the implications all that much. It can be effective, but it is ultimately out of place in a book that includes a Doom-style first person shooter game scene. As in, they kill something that drops shotgun shells, and a nearby crate has a shotgun in it.
Fun, engaging for a couple of nights, and it should make an entertaining movie.
Amazon link