Andrew Vachss has reinvented detective fiction for an age in which guilty secrets are obsolete and murder isn't even worth a news headline. And in the person of his haunted, hell-ridden private eye Burke, Vachss has given us a new kind of hero: a man inured to every evil except the kind that preys on children.    Now Burke is back, investigating an epidemic of apparent suicides among teenagers of a wealthy Connecticut suburb.  There he discovers a sinister connection between the anguish of the young and the activities of an elite sadomasochistic underground, for whom pan and its accompanying rituals are a source of pleasure—and power