Down in the Zero

   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 discov