Try to imagine the scariest, most terrible movie you've ever seen. Recall the sequences and events that made you squeamish or induced bouts of barely held back nausea. You know what I mean: the sound of a door creaking or slamming open/shut, or the big scare when the monster/villain pops up from the around the corner/out of the floor/ceiling/back from the dead, or the ending of a survival horror movie, where the hero survives, but is given a huge scare right at the end, with a cliffhanger ending and huge crescendo, or the part where the good guys get covered/submerged in filth/insects/blood and guts. Now imagine that instead of seeing this on a screen, it's happening all around you, and much to your own silent horror, to you.
Next try to remember the funniest thing you've ever read, watched or heard. Be it Shaun of the Dead, or an episode of It's Always Sunny/any clever sitcom, or a clever treatise by a favorite humor writer. Or a dirty joke you heard from a friend, or a funny memory that can't help but make you laugh everytime you remember it.
Now superimpose the two extremes together, to form a mold of crazy horror and black comedy. Because John Dies at the End, by David Wong, is more insane, scary and hilarious than what you've just imagined. In fact, I dare say that this book freaked me out as much, if not more than 'Salems Lot, The Stand, and Pet Sematary. And it made me laugh out loud with David Wong's usage of ass metaphors and crazy similes and silly puns (you have to read the part about the chair).
The closest great writer that I am reminded of when reading JDatE is Douglas Adams of Hitchhiker's Guide To The Galaxy fame. And Goddamn if that isn't high praise. Get this book.