Man's Search for Meaning is a 1946 book by Viktor Frankl chronicling his experiences as a prisoner in Nazi concentration camps during World War II, and describing his psychotherapeutic method, which involved identifying a purpose in life to feel positive about, and then immersively imagining that outcome.
In Man's Search for Meaning, the capos were prisoners who were chosen by the SS guards to oversee other prisoners. These privileged individuals were picked for their sadistic qualities and could often be just as brutal as the SS.