I read The Confusions of Young Torless by Robert Musil
Robert Musil was probably one of the best to ever do it, but up until now he was kind of a hard sell for me to push on other people because, well, his most famous book is over 1000 pages long, and about the doomed attempt to plan a festival celebrating Austrian culture on the eve of the first World War.
And I do still recommend The Man Without Qualities! However, that this book is only around 160 pages long, and keeps the psychological density of its characters and the level of philosophical questions raised by the text intact, makes it even easier to tell people to go and read.
It's very moody and amoral, which will be familiar to anyone who has read the sections of The Man Without Qualities that stick close to Ulrich's cool and distant viewpoint. The character is in a sort of similar position; not the most morally compromised bully, but not wholly targeted by them either, sometimes a distant ally, sometimes threatened if he seems to be pulling away from their influence. He becomes a part of a scheme where two other boys come up with increasingly sadistic psychological and physical humiliations to target another boy at their school, an outcast who was caught stealing other students' pocket money.
Instead of just being a direct allegory of how a disinterested spectator can come to go along with pointless brutality, Musil is more interested in exploring how these sorts of situations are in turns interesting, (sexually) exciting, confusing, repellent and back again. The subtlety of changes in emotion and perception between the characters is really masterfully noted, beat-by-beat and really emphasized to me why Musil is described as "a writer with an engineer's soul."