After years of having the novel on hand I finally finished it. There's not much difference between it and the anime and is actually the weakest of the three tellings to me. A hikki simply wanted to make something and the result is understandably amateurish, Welcome to the NHK is still popular because it's the only anime/manga that explicitly is about hikikomori. Anyway this felt like a prototype for the manga and anime, and a lot of light novels seem to be. For starters it felt short, not much happens across its 220~ pages, it felt empty. The whole book is written tersely in that the plot's flow is choppy with scenarios feeling stitched and not as a stylistic choice to represent listlessness. You don't really get a feel on Satou's actions and behavior either, it's mostly about what's going on around him and that is told in just basic descriptions, what internal monologue about him is shown is just about how useless he is which worked in the anime and manga as he was partially a subject we were observing but literature does not translate that well when your book is in first-person. Here are some plot differences.
>Satou drops out of developing the game instead of sticking through and it happens fairly early in the story, in general he comes off as cooler-headed and apathetic compared to his agitated and restless state in the manga and anime
>drugs
>Yamazaki gets hooked on loli and drugs much deeper than Satou
>the vocational school is only mentioned, it is never attended as Yamazaki is skipping
>Satou and Yamazaki don't go to Akihabara
>Nanako isn't included
>Satou's parents are only mentioned and in relation to his allowance
>Satou meets Kashiwa just once
>I don't remember in the anime if it's mentioned but Satou and Kashiwa fucked once in high school
>Misaki seems meeker and less antagonistic than in the anime
>her past and family life is left vaguer
>the religion her aunt belongs to is Christianity, in which Satou and Yamazaki attend a service
>the ending is less definite to if he'll stop being one at least any time soon
What I liked about it is that there's no deviations like in the manga and anime where Satou impulsively falls for a bad habit he traps himself in (or scheme in the case of reuniting with the class rep) that you'd expect him to be smart enough to not fall for after getting hooked on porn, like it's a fucking villain of the week. That made up a good portion of the other two so it seems like cognitive dissonance I'd complain about that but it halted any chemistry I felt was occurring by throwing in a sidetrack to display hikki activities like MMO addiction or maybe I'm bitching too much and that it's realistic he wouldn't learn his lesson so easily, I didn't mind it in the manga so much.
Also the English translation was definitely done by an ESL still learning to the language, I wonder if it felt so skim because some parts may have just been fucking cut out. It's fucking astounding that this saw publication.
Manga > anime > novel. The best version exists somewhere between the three of them but the anime usurps the novel, it was like a weaker version of it.