What is the LITERATURE equivalent of Welcome to the N.H.K?
Yes, I know it's a light novel. I want LITERATURE.
There is no equivalent.
The closest you'll find is probably A Man Asleep by Perec.
Notes from the Underground, No Longer Human, etc. is pushing it.
If you're into crazy monologues fuelled by despair and solitude, Thomas Bernhard's novels are full of them.
It's not capital "L" literature but i got some NHK vibes from "Colorless Tsukuru Tazaki and His Years of Pilgrimage"
Yeah, the hikkikimori phenomenon is too new a thing to have been represented in older literature. There had been shut-ins in the 20th Century, and eccentric layabout aristocrats before, but not really anything exactly anything like there is now (I suppose they didn't have the same kind of [Japanese] consumeristic infrastructure and online culture to feed and support it).
Equivalent is too hard of a word like >>7314276 said.
Notes from the Underground comes close due to its sense of humor. À Rebours could be of interest too but the Mc's motive is completely different.
Tolstoy has a 2 or 3 short stories centered around eremites but the tone is completely different too.
Regarding delusional paranoias, maybe check out The Trial by Kafka.