Books where the main character goes insane?

I go to the Eastern library in Iowa and I'm wondering where I could find books like that? I'm talking about when the MC develops a mental illness or condition (I don't really care if it's a fictional condition or not) and they become insane because of a lack of help or support. I'm not looking for specific authors.

And the book has to be fictional and I prefer genres like horror, mystery, and thriller.

Sorry if this comes off odd or weird, just please don't dogpile on me or anything.