Asklemmy

The one that included the most wildlife might be hard to know exactly, but 'The Lost World' by Arthur Conan Doyle might be a contender.

One of my favourite books, and one that gave me lots to think about was His Dark Materials by Phillip Pullman.

The most 'different' setting for a book that I've read might be The Planiverse by AKA Dewdney, which takes place in a 2d world with thought out and realistic physics and societies.

NOFX: The Hepatitis Bathtub. Wildest because it’s an autobiography, and they spill it all.
Edit: find the audiobook if you can

Currently reading The Illuminatus trilogy. It is a trippy, psychedelic thriller, which assumes many conspiracy theories, both well-known and obscure to be true.

Dhalgren by Samuel Delany

(Close race with House of Leaves, but ultimately House of Leaves was telling a specific story, whereas Dhalgren is a semi-incoherent drug trip. Loved both books, though.)

In elementary school I read this book called "Flawed Dogs" and it was unforgettably wild. It's about a dog who escapes some kinda confinement by jumping over a barbed wire fence and loses his back legs in the process, and then joins a dog gang and does dog gang activities. Also one of the dog gang members was a cat in disguise.
Honestly I should see if I can find a copy of it and reread it. It was pretty wild.

edit: I looked it up and maybe I have a lot of the details wrong but it's still pretty wild

I don't know about wild, but UNSONG has been a very weird trip. It's like science fiction, except instead of science its Jewish kabbalah. There's angels, demons, alt history American politics, religious references that are truly esoteric, and puns... lots and lots of puns.

Santa Steps Out was wild.

'Sex, Death, and Santa Claus

His generosity is legendary. He has a devoted wife, a crack team of sky-borne reindeer, hordes of industrious elves, and the love of good little boys and girls around the globe. But what unholy desires now propel him into the lascivious clutches of a certain fairy? And who was he before the sleigh and workshop, in times forgotten?

She munches on molars, summons drowned sailors to her pleasure, and recalls, sharp as a pinprick, her life as the most savage of ash nymphs. Why then is she stuck, night after night, hovering above pillows to leave coins for gap-toothed brats? More important, how quickly can she captivate the jolly old elf to the north?

He's huge, fluffy, lonesome, and unbearably horny. On his Easter rounds, he contrives, as often as possible, to get a grip on himself and peer into interesting bedrooms. But who in the world will throw him down and ravage him as the lovers under his gaze ravage one another?

Deadite Press is proud to bring back the ultimate erotic Christmas story from Robert Devereaux'

"The teachings of Don Juan" by Carlos Castaneda. Read it in highschool and it put me off psychedelics for more than two decades.

Fanged Noumena by Nick Land

Cyclonopedia by Reza Negarestani

Both are a naked lunch level mindfuck. Don't treat it as a book, but rather as a stream of consciousness on the acid trip. Don't try to make any sense, just ride the wave.

Probably some short story I read in high school but from what I can remember the first one that came to mind is Blood Meridian

Clockwork Orange

A long way gone by Ishmael Beah was pretty dark. Story of a boy soldier from Sierra Leone explaining how you get forced into it and the terrible things they did.

Kitchen Confidential.

Depends in what way you mean 'wild.' Crazy even psychedelic, but nonetheless benign? Or are we including disturbing?

Sadly, Porn

I don't know how to describe it, expect to be confused and offended and gaslit.

The multiorgasmic man.