Looking for any book that fits loosely along the lines of “Character becomes more and more obsessed with something, or becomes slowly mad, or goes through some kind of severe mental break.”
Preferably fiction, any genre works. You can provide more details if you want but please put them in spoiler tags!
A popular, well-known example would be Stephen King’s >!The Shining !<though it doesnt need to be horror
by _BreakingGood_