BROWNSVILLE (Back bay, $13.95) is a slim but just about perfect debut story collection by Brownsville, Tex., wonder boy Oscar Casares. His characters are border-town castaways with one foot in Texas and the other in Mexico; their stories, likewise, are Tex-Mex dramedies, sad and yet very funny. In ''RG,'' the narrator seethes for three years, 10 months, and six days after a neighbor forgets to return his hammer. In ''Chango,'' Bony, a lonely 31-year-old hombre still living at home, faces resistance from his family when he befriends, quite plaintively, a severed monkey's head. ('''¡Ay, Bony!' his mother said. 'No me digas que you brought that chango inside my house.''') That curlicue on the book's cover is, in fact, a bushy monkey's tail, a fitting reminder that the terrific stories it teases are warm, exotic little ticklers. Casares' bio says he's at work on a novel. ¡Andale! ¡Andale!