Controversial, modern, and unafraid to look the reader straight in the eye, ''Serpent's Tail Classics'' are books for life. Cult American noir is now a ''Serpent's Tail Classic''. The depression of the 1930s led people to desperate measures to survive. The marathon dance craze, which flourished at that time, seemed a simple way for people to earn extra money - dancing the hours away for cash, for weeks at a time. But the underside of that craze was filled with a competition and violence unknown to most ballrooms. A lurid tale of dancing and desperation: Horace McCoy's classic American novel captures the dark side of the 1930s.