What are some books that have the main character gradually descend into madness throughout the narrative...

Unironically this. I wrote my first book, Serotonin Deficiency, two years ago and towards the end the character says "I'm terrified of spending my life in a mental institution." Two weeks after I made it public for people to view, I was involuntarily hospitalized and diagnosed with paranoid schizophrenia. I felt it added artistic integrity to the novella, and, when I read it again, the knowledge of my psychosis is the only way the ending makes sense, because I was already pretty psychotic at that point.

Can second this, also "Death of a Salesman" by Arthur Miller, if you mean a behavior that causes suffering or poor ability to function.

To the White Sea

It's excellent, and it's by the author who wrote Deliverance.

No, the madness of Grendel sets in quickly and reactively to the unfathomable nightmare that Beowulf is. Prior to getting his arm ripped off by a harbinger of God, Beowulf is pretty sane, just heavily ostracized and unable to communicate.

Euripides' Bacchae

Dom Casmurro

>Hunger.
Was just about to say. He even starts at madness and delves deeper into madness, and the writing is great

t. Norwegian

What did you think of 'growth of the soil', I mean especially compared to these? I couldn't even finish it

Not really madness, more moral/social disintegration. Same goes for the Immoralist.

gogol's diary of a madman

The quentin chapter in The sound and the fury is pretty much the ramblings of a depressed man at the verge of a suicidal episode.

unironically, though not as gradually and with several characters, this

Dead Souls by Gogol

He lost his shit while writing it

The Outsider starts off with a guy that just doesn't give a single fuck and he quite quickly becomes irrational when he forces himself not to notice the meaninglessness of his life.

don't smoke at ur ma's funeral
or drink a white coffee. ever.

American Psycho? : ')

The New Testament.

Strange Case of Dr Jekyll and Mr Hyde

Open the pod bay doors, Hal

I mean, he's crazy p much off the bat, but it gets worse throughout the book.

...

Temple of the Golden Pavillon

Hærværk by Tom Kristensen

Danish novel that depicts a writer who one day quits his job to become a drunk.

Also the novel that got me into litterature

Dostoyevsky's The Double makes me feel insane while I read it.

The Stranger and One Flew Over the Cuckoo's Nest sort of fit your criteria.

Great novel!
Also Berlin Alexanderplatz.

the yellow wallpaper did a really good job in a short period of time

The second part of Molloy