KENT THOMPSON`S fifth novel, Playing in the Dark (Quarry, 139 pages, $12.95 paper), is an intentionally brutal, ugly, and shocking story about a pair of brutal, ugly and shocking misfits: Stringy, an aptly named drifter and petty criminal, and Sharon, a young bank teller who knew Stringy when he was a child and now becomes his girlfriend. The problem is that Stringy and Sharon, and for that matter nearly every other character in the book, are so emotionally dead, so repulsive, and so predictably vile that it is hard for readers to muster up much concern or interest in them. Thompson seems to intend that we, his presumably comfortable middle-class readers, should enter into the interior lives of Stringy and Sharon without actually liking them. In this he succeeds all too well -- it isn`t difficult to decide that we are better off avoiding their company.