The book Totally Wicked! has worm-like monsters that suck out the insides of both humans and animals. It's even worse when the main characters find the skin of a dog floating in the wind.
Speaking of the other Wicked, it has several examples, the most prominent of which being that not only is Elphaba born green, she is also born with vicious teeth. She likes biting, and has to be restrained to stop her hurting herself, and can't be breastfed.
Ron's unfortunate "splinching", where a chunk of his arm is missing.
"Bertie Bott's Every Flavor Beans", some of which taste like boogers, vomit, or even earwax. Their real-life counterpart (made by Jelly Belly), contain regular flavors as well as grosser ones like Spinach, Spaghetti, Grass and yes, Boogers, Vomit and Earwax. They're actually pretty spot on.
Apparently, the (as-yet unrevealed) ritual for creating a Horcrux acted as nausea fuel for J. K. Rowling's editor. Rowling has said that the ritual will be spilled to the general public in the Harry Potter Encyclopaedia when it eventually puts in an appearance.
The description in Making Money of Cosmo Lavish's gangrenous, rotting finger (thanks to wearing a much-too-small signet ring described as cutting off his circulation entirely, because he desperately wants to imitate Vetinari) complete with the descriptions of the rotting stench of his hand stewing in its glove and his erratic behaviour as it appears to poison him.
Speaking of Pratchett, in Wyrd Sisters, when Duke Felmet keeps trying to wash the blood off his hands, eventually grating them until they're nearly completely gone and have to be continually bandaged. To wit — when we say wash, we really mean "ground off to a bloody stump".
In Breaking Dawn, the part where Bella's vampire Death Baby is being born: it breaks her spine and ribs, and then Edward performs a Vampiric C-Section with his teeth, and chows down on the afterbirth.
The sequence in Haunted in which one character is believed to be dead, so the others cut off pieces of her flesh to cook. She isn't dead after all, and winds up eating some of her own flesh before she does die.
The Things They Carried by Tim O'Brien falls into this through brutal honesty, going so far as to tell the reader that if you would censor the word "shit," you would censor the truth. That doesn't mean the death of Kiowa is any less disgusting — he drowns in excrement.
K.A. Applegate's Remnants series was full of this. From the first book with the horrible fungus-filled cryopods of character's family members, to the live flesh-eating worms, to the guy with all his skin painstaking burned off and replaced with a clear coating...
"Turn Coat" has the scene where Harry is describing sensing Shagnasty with his Sight activated.
"Try to imagine the stench of rotten meat. Imagine the languid, arrhythmic pulsing of a corpse filled with maggots. Imagine the scent of stale body odor mixed with mildew, the sound of nails screeching across a chalkboard, the taste of rotten milk, and the flavor of spoiled fruit."
The rampage of the loup-garou in Fool Moon is one of the most explicitly violent scenes in the series. It seems as if everyone in the station is horribly killed or traumatised for life.
Annie crushes a rat to death with her bare hand, poking her fingers into its body in the process. And then she licks her fingers. Paul wasn't the only one that felt ill. Also, sometime during the later half of the book, Annie cuts off one of Paul's thumbs. And then puts it on a birthday cake and serves it to him, making a not-so subtle threat to force him to eat it.
The part where she forces him to drink her filthy cleaning water. As well as the part where she hacks his foot off with an axe and then cauterizes the wound with a blowtorch. Sounds awful, turns out to be far, far worse.
In "Gray Matter," we have the charming tale of a man who is transformed into a fungoid blob caused by a bit of gray slimy guck infesting one of his nightly cans of beer. Later, we hear his son's story of seeing what's become of his father eating the bloated, putrefying, maggot-ridden corpse of a cat.
In "The Lawnmower Man," the title character's self-driving machine runs over a mole (an analogue of an earlier scene where the same thing happens to a cat, albeit with a different mower). The man, following close behind, eats the body of the mole just as he has been eating the expelled grass clippings.
"The Mangler." Imagining a person crushed and folded by an industrial laundry's steam ironer (and having their remains taken out in a basket) is enough to make someone ill.
"Survivor Type" in King's Skeleton Crew has our main character, a disgraced surgeon, stranded by a shipwreck on a barren lick of rock in the middle of the ocean. When he breaks his ankle chasing a wounded seagull, he is forced to amputate it. Guess what he does? And it doesn't stop with just his foot....
American Gods has Laura, the main character Shadow's undead wife who slowly rots over the course of the story. It starts with Shadow french-kissing her dry formaliny mouth and ends up with her coughing up gobs of maggots. Complete with description of how that feels to her. Thanks for that Neil! The same book also features a man-eating vagina. Quit sniggering.
Filth by Irvine Welsh features a disgusting detective who has a tapeworm living in his guts. It has some horrendous, graphically written scenes of the tapeworm coming out for a visit. Also, the guy himself does some pretty vile things throughout the book.
Friday the 13th: Church of the Divine Psychopath spends an inordinate amount of time describing how horribly mangled and infected a character's foot has become. It goes From Bad to Worse when he tries running from Jason, and steps on a really pointy stick, then Jason impales him, rips the machete out, lifts him up, and jams his hand into the machete wound and starts pulling stuff out.
Princeps' Fury, the fifth book of Codex Alera, briefly features a Taken grass lion rotting alive, complete with description of maggots squirming under its flesh.
Any edition of the Guinness World Records released since circa 2000. It was around then that it seemed to turn into Guinness Book of Gross-Out Records, with as many disgusting images they could manage per page.
American Psycho, sticking a tube with a rat in it up a woman's vagina then chainsawing her in half, and using a wide drill bit to gouge a woman's mouth open, skull-fuck her, then pull her throat out through her mouth. Or where he nails his ex-girlfriend to a plank of wood and destroys her body while she is still alive.
Any book documenting torture devices through history.
In The Naming of the Beasts, Felix Castor attempts hand-to-hand combat with the demon Asmodeus.
He kicked something across the floor at me - something red and wet that looked as though it belonged in the little plastic bag you find up a chicken's arse and throw away before you cook it. Part of my jawbone; I could tell by the fact that it still had three teeth embedded in it.
Le Nausea by Jean Paul Sartre. Pretty much coined the term "Existential Nausea".
Antoine : The Nausea spreads at the bottom of the viscous puddle, at the bottom of our time—the time of purple suspenders and broken chair seats; it is made of wide, soft instants, spreading at the edge, like an oil stain.
A Japanese folktale tells of a certain prince who was insulted that a certain highly intelligent princess refused to see him. After she scares him with a mechanical snake he steals her chamber pot to prove she isn't perfect. After inspecting the pot thouroughly he drinks its contents — the princess had filled the (presumably brand-new) pot with golden tea and fragrent herbs.
A Japanese folktale has a monk who has "relations" with his young acolyte, who turns out to be a woman, who is really the goddess Kanon. When she goes away she leaves a giant nugget of gold (the monk thought his lover was pregnant), as well as a bucket of all the sperm from their relationship.
Jeremy Robert Johnson's short story compilation Angeldust Apocalypse, in which a character takes horse tranquilizers and begins to cut disembowel himself with a scalpal, pulling out his intestines while his parents watch in horror. And that may be one of the tamer stories in the book.
The original French version of Frédéric Beigbeder's award-winning novel, Windows On The World features one of the squickiest sex scenes in literature. An adulterous couple on one of the upper floors of the South Tower of the World Trade Center having sex is probably questionable enough. The author, however, decides to turn up the squick by having the lovers surrounded by poisonous smoke (they're called "asbestos lovers" at one point during the scene), purple corpses of their former co-workers, and water levels reaching up to their thighs. The scene was so unsettling that it was removed in the American translation of the Novel.
Someone Else's War: Matteo finds out that children who try to escape the Lord's Resistance Army are first killed, then cut into pieces, then served to the rest of the children along with their dinner.
In Kafka'sThe Metamorphosis, Gregor-turned cockroach Samsa is described as being covered in wet dust, having a rotting apple stuck to him, and leaving a trail of slime on the floor. And he eats garbage. Eww.
Children of the Lamp has a spell that causes whatever a person would least like to eat to come out of their mouth. Luckily the protagonist we see get hit with it REALLY hates vegetables, so that's all we see, but the djinn who used the spell on him says that for other people, it's been things like frogs or bugs. And the description causes the reader to ponder what it would be for them...