I recall reading as a kid any number of mystery stories where the protagonist was shut-in for some reason (agoraphobia, for example), and misanthropes abound in anything with a hint of noir, but introverts per-se seem to be rare (which strikes me as odd, since authors are often introverts).
So, any recommendations? I'm personally interested in science fiction and fantasy, but mysteries or (techno)thrillers will do as well. Not really into horror, but don't let that stop you, I'm sure others here will be interested.
Haruki Murakami's narrators all seem like introverts to me. Try Colorless Tsukuru Tazaki or 1Q84.
Many of Iain M. Banks' characters are. Try The Algebraist or The Player of Games.
In Margaret Atwood's The Handmaid's Tale, the main character keeps mostly to herself, but I'm not sure if that's due to her introversion or the broken world she lives in.
She's a genki girl in the anime (hyperactive trope) but only when she is alone. When she talks to others, she's an introvert (very terse and soft spoken), which is an interesting version of the trope. The only name she goes by is Kumoko (rough translation: Miss Spider, or maybe Spider-cutie).
In the original source material (the original Novels), she's fully an introvert. But I think the problem with anime is that acting in front of an audience is innately an extrovert activity.
Introverts can be side characters in an anime (Rei from Evangeleon, Nagato from Haruhi), but since they sit around and think to themselves so much it's hard for them to be a main character. Kumoko is an interesting way of doing it, since she is only hyperactive when she is alone.
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Mild spoilers: but the people who got transferred over got forms that best represented their soul. Most became a human again, but a few swapped genders and another few became monsters.
In the case of Kumoko, her introverted nature matched the spider the most. In contrast, teacher has the 'young at heart' trope and thus the teacher got the form of an elf, for example.
As an Isekai novel / anime, it's fantasy + sci-fi. They are all clearly in some kind of techo-video game world, but magic seems to have been the cause for how they all got trapped in the first place.
Without wanting to spoiler you — it’s about a few introverts, and it’s narrated in a way that focuses heavily on the inner world, on their thoughts, the culture they enjoy, their deliberations, what they say and what they hold back.
It is a horror too, sort of.
bapublications.com
Also, Kafka’s protagonists could reasonably be be considered introverts. Their dialogue externalizes the feeling of being introverted, in that they acknowledge futility, and they produce intricately detailed considerations that others are unable to relate to entirely, but it is presented as though there is no reservation in stating their inner concerns outright. Kafka, in this way, presents an excellent study of introversion if introverts had few, if any, inhibitions. This often results in the grotesque.
The protagonist is definitely an introvert, and her narration is basically what made the story work for me. It's YA, but without too many tropes.