The book that Bobby's mother reads before he enters the room is Jean Paul Sartre's "The Flies" and the cup Bobby brings her says "Lord what fools these mortals be!" a line from Shakespeare's "A Midsummer's Night Dream."
The graffiti on the pillar that Otto leans against in the second half of the films reads "Crime". It also fades in and out of focus in the second to last scene just before the vigilante walks away after sparing Bobby.
In the first scene, the books lying on the table reveal information about both the character's situation, and future events. On the table are different medications and several books. The titles say "Our Town" by Thornton Wilder, "Dictionary of Symbols", and "The Alienist" by Caleb Carr. The latter depicts a shadowed figure in a frock walking down a city street, and centers around the search for a serial killer during the 1800's. The image foreshadows the Vigilante's entrance in the second half of the film.
The line that the vigilante utters to Bobby was originally from the nursery rhyme "A Ring around the Rosie" referring to both the pink bracelet around Bobby's wrist as well as the Plague doctor himself who's appearance is modeled off of medieval plague doctors during the Black Plague. When editing the film's sound the creative director suggested deliberately making the Vigilante's words unintelligible thereby putting the viewers into Bobby's perspective and forcing them to make their own conclusions as to what was said.