Slaughterhouse-Five revolves around Billy Pilgrim, who like Vonnegut himself, was a POW in Dresden when it was firebombed during World War Two. Billy has “come unstuck in time.” And to complicate matters further, he is abducted by aliens, two-foot-tall creatures shaped like toilet plungers, from the planet Tralfamadore. It's been challenged at least eighteen times, primarily for “obscene language,” and “anti-American” sentiment. And yes, it was actually burned as a result of one of those challenges.