In the figure of Burke, Andrew Vachss has given contemporary crime fiction one of its most mesmerizing characters. An abused child raised in orphanages, foster homes, and prisons, Burke is a career criminal and outlaw who steals and scams for a living.    In Blossom, an old cellmate has summoned Burke to a fading Indiana mill town, where a young boy is charged with a crime he didn't commit and a twisted serial sniper has turned a local lovers' lane into a killing field. And it's here that Burke meets Blossom, the brilliant, beautiful young woman who has her own reasons for finding the murderer—and her own idea of vengeance.  Dense with atmosphere, savagely convincing, this is Vachss at his uncompromising best.