Der Prozess (1915) by Franz Kafka, it still is relevant today.

The Wind-Up Bird Chronicle by Haruki Murakami just a magnificent read, you probably couldn’t go wrong with any of his works.

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.

Sadly, Porn

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

The Fan Man

The multiorgasmic man.

Definitely House of Leaves. A story inside of a story, inside of a story, with all narrators being just a bit crazy. Text of different fonts, going all over the place and even upside down based on the story. Just make sure to get the physical copy.

Not a book, but a webcomic: https://elan.school/

Be careful what you wish for OP, this is THE WILDEST shit you will ever read (at least top 5, guaranteed) and the worst/best part is that it's all true.

Also, its VERY addictive so clear your schedule.

You've been warned.

You've ALL been warned.

I went into Do Androids Dream of Electric Sheep? blind. Hadn't seen the movie, hadn't read any other Dick, hadn't even had it hyped to me by a friend. What a series of mindfucks.

Fear and Loathing in Las Vegas is an obvious but nonetheless relevant answer. What a ride.

Also Infinite Jest.

The Book of Rack the Healer by Zach Hughes was pretty wild.

It's 'New wave' sci-fi from the 1970's, and revolves around these mutated humans in a deeply poisonous and radioactive world where it's forbidden to dig into the earth.

The humans have evolved a carapice and internal air sacks that they fill to hold their breath before leaving their safe organic dome homes that change color depending on their mood. Some of the domes have women in them that don't seem capable of complex thought, and live purely through sensory input, are telepathic, and are basically constantly edging themselves all day.

It's a drug fueled fever dream, for sure.

Strange new world by Heinlein.
Martian Jesus comes back to earth and is like, wtf guys?

I’m not sure if it’s the wildest but the first that comes to mind is “John Dies at the End”

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.

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

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

Kitchen Confidential.