….And I found it in my high school library. It was newly released I think, so it must be six years old or so? Give or take a year or two on either side
It was this incredibly, indescribably creepy story about a young (pre-pubescent) girl living in a nightmare-version of government sponsored housing, a huge tower with individual apartments. If I’m remembering right, nobody was allowed to leave. The entire story took place indoors, and all of the characters, except a new resident and maybe the protagonist were horrifying, terrible people. The protagonists was the only kid mentioned, she didn’t have any parents or guardians, and survived by trading items/favors/information with the other residence.
There was a murder, and she was looking into it, I believe. There was a sub-plot about cannibalism in there somewhere, and the big climax of the book was this man (who the protagonist knew and did business with) carving a possibly Japanese-inspired design into her chest with knives and saltwater. Prior to that, he’d seemed like one of the nicer/saner characters.
At the very end, there’s a scene with the protagonist showing the new resident (she was a social worker if I remember right) how the business of trading worked in the tower and the new resident gave the  protagonist jelly beans.
I think the protagonist’s name was Jem. But don’t quote me on that.
It was the most disturbing thing I’d ever read at that point in my life, and I’ve never been able to find it again.