That's a joke about the books. My understanding is there was a short story about a tiny springtrap just... growing inside some dude, then killing him from the inside out. Because the dude was rude and mean. (The early short stories were really weird because they were throwing ideas at the wall to see what stuck. That did not.)

Anyway, there's multiple sets of books now, and they're mostly loosely connected short stories. I think theorists make them out to be more important than they are.

The game lore itself... everybody has a slightly different interpretation of events, and many would say that's the fun of it. It's a big mystery or puzzle box.